Haven't posted in a while, but I came across this thread and felt the need to interject a my thoughts and opinions. Please forgive me! I suspect that anyone wanting to learn musical form really just wants to learn how to be a better composer. If somebody asks about musical form it's probably because they believe their lack of knowledge on the subject is hindering the effectiveness of their compositions. They probably also believe that writing with the knowledge of the various musical forms will help focus their composing and bring a stronger sense of purpose to their music. This would all be true, in a sense, but it might also be totally missing the point.
I quote a passage, translated by Susan Gillespie, from an excerpt of Gustav Jenner's Johannes Brahms als Mensch, Lehrer, und Kunstler (Marburg, 1905). Jenner was a composition student of Brahms for a short time, supposedly the only one, this excerpt is during one of his lesson:
"But then he went over the trio and the songs with me all the more exhaustively. At the first movement of the trio there was much turning of pages back and forth. With devastating precision Brahms demonstrated to me the lack of logic in the structure; it was as if in his hands the whole thing dissolved into its component parts. With growing horror I saw how loosely and weakly they were joined together. I realized that the bond that was supposed to hold them together was less an internal than an external one; it was nothing more than the device of the sonata form. The essence of form began to reveal itself to me, and I suddenly realized that it is not enough to have a good idea here and there; that one has not written a sonata when one has merely combined several such ideas through the outward form of the sonata, but that, on the contrary, the sonata form must emerge of necessity from the idea."
Here's a link to another translation of some different parts of the book, a fun and short read:
http://davidsbuendler.freehostia.com/jenner.htm
So I gleam from this little passage the thought that knowing sonata form will not necessarily make you a better composer versus someone who doesn't know sonata form. Knowing how to write an exciting 1st theme group, and coupling it with a beautiful 2nd theme will not yield a dramatic and purposeful exposition. Each of these themes could be expertly written music, but if they don't demand the necessity of each other they are simply component parts of sonata form. A 1st theme group must necessitate the 2nd theme, the 2nd theme demands the closing group, the exposition must require a development, a continuous chain of necessity that makes sonata form an emergence and not simply a formal structure to be recognized as existing. The parts of a sonata form are not "muffin tins" that are simply filled in with the right material, baked to 350 degrees, yielding a full-fledged beautiful sonata; and yet, this is exactly what is taught in so many forms books.
The books will teach you to recognize various forms by identifying their component parts. They'll show you how various forms grew in complexity as the component parts became more intricate. They'll make generalization about form that will help you anticipate formal structures based on period and genre characteristics. In other words, they teach form from the outward perspective, completely missing the whole point, that form must emerge of necessity from the idea. Form is an inward process that is composition specific! What these books won't do is thoroughly examine piece after piece and explain to you why a particular 2nd theme, and no other, must go with said 1st theme. They won't explain to you why a particular 1st theme purposely under-develops some part of itself to necessitate a specific 2nd theme. You can only get this kind of knowledge from mercilessly examining the masterworks, and asking the right questions!
I've been talking specifically about sonata form, but all forms are really the result of this inner emergence of the necessity of the idea (motive, phrase, theme, whatever you want to call it). Simply knowing the common musical forms/structures and writing within their bounds will not make you a better composer from someone who doesn't know musical forms. You can only write a good theme and variations with the proper theme, any old theme will not do. A perfectly suitable 1st theme for a sonata form may not be suitable in a binary form work. Any old subject cannot be turned into a good fugue simply because you know what a fugue is. These decisions are unique to each composition! You must realize that when you look at a completed masterwork, you are seeing the fully-flowered emergence of the form from the outside perspective. What you don't see is the inward process of how the composer meticulously crafted the interlocking components (from left to right, from beginning to end) that causes the form to emerge as the work is performed from the 1st measure to the last measure. This is the true essence of form! I know that sounds ridiculously grandiose and pompous, but I really believe Brahms was right.
Now when I said you can only get this knowledge by examining the masterworks, that is partially true. There is no substitute for a thorough, focused, independent analysis of the masterworks, but it is also true that an analysis will not provide meaningful answers unless you ask the right questions. So I propose a course of study below, not an easy one by any means, but one that I think will help a person arrive at the relevant questions. The questions that will ultimately lead to becoming a better composer.
1) You must get a solid grounding in tonal theory. It is not the end all and be all (as there is so much wonderful music out there that completely ignores the tenets of tonality), but it is pedagogically the easiest starting point. I recommend: Laitz, The Complete Musician; or Aldwell & Schachter, Harmony and Voice Leading; or even Koska & Payne Tonal Harmony is ok. A warning, these books have the outward perspective flaws on composition like so many others, but sometimes you have to learn something slighty wrong first in order to have the foundation to learn it right later on.
2) Once you have the basic tonal foundation you must go through the Fux, Gradus ad parnassum. There is a suitable cheap English edition by Dover Publications. This will seem like a step backwards, but rest assured it is not; an indispensable introduction to good counterpoint. Brahms would have approved!
3) Go through C.P.E. Bach, The True Art of Playing Keyboard Instruments. This is an absolutely fascinating read, with insights into composing, learn improvisation through figures, kernels of compositional truths and exercises handed down to C.P.E. by his father J.S. Bach.
4) This is where my pedagogical approach will no doubt elicit some loud and understandable disagreements with others. I have caveats that go along with this, but I feel there is more positive to be learned here than negative. You must read Schenker, Counterpoint (Books I and II, translated into English by John Rothgeb). Heinrich Schenker is clearly an influential, hotly disputed, and misunderstood figure in the music theory world. Schenker's theories have flaws, but no other theorist has ever come close to Schenker's reductive analysis approach. The major caveat is: you must have all your senses fully alert when you read Schenker, and do not believe everything he writes. You will not fully understand what he is talking about on a first reading of his books. You will have to read his books many times over before you will understand what he is getting at.
5) Go through Schenker, Free Composition (2-volume set, text and examples translated into English by Ernst Oster). Another Schenker book that you will have to read many times over. For now it is sufficient to say that Schenker didn't invent reductive analysis, but this book really shows that his multi-temporal approach to reduction is unique and profoundly eye-opening. Reading Schenker is where I realized that music is not simply linear, but operates on multiple layers of time. It not enough to say that composing music is organizing sound in time. Composing music is using sound TO ORGANIZE TIME! Once you realize this extremely important truth, you'll begin to see how a musical idea can create necessity by presenting itself and interacting with itself across multiple time scales (or measurements of time, what Schenker refers to as Background, Middleground, and Foreground). The necessity of this interaction causes the form to emerge, not from an outward process, but from the material itself, as Brahms instructs. An appropriate analogy might be the emergence of a complex and beautiful fractal that comes from a very basic algorithm. Of course, fractals organize space as their algorithms emerge, but a composition organizes time as its form emerges. Remember, be suspect of everything Schenker writes, but keep an open mind when reading. He shares more in common with Brahms than most realize.
6) Optional, read Schenker, Harmony (translated into English by Oswald Jonas). This was the first book writiten in his three-part theory series (Harmony, Counterpoint, Free Composition), but I think it is best understood when read last.
7) Finally, a few more reading gems I'd recommend: Ledbetter, Continuo Playing According to Handel and His Figured Bass Exercises; Renwick, The Langloz Manuscript-Fugual Improvisation through Figured Bass; and Salzer & Schachter, Counterpoint in Composition. Also, if you find Schenker's Free Composition too difficult to read at first try: Cadwallader & Gagne, Analysis of Tonal Music; and/or Salzer, Structural Hearing-Tonal Coherence in Music.
In conclusion, one must not simply read them, you must do the exercises in them, play them at the piano, do the realizations, take a particular concept and explore it with a composition of your own material. You must write a lot and not always keep what you write, as Brahms instructs Jenner, but you will just spin your wheels if you don't understand what is yielding your poorer work, and trust me, it is not simply a lack of knowing musical forms. Examine what the masters do, armed with a thorough knowledge of emergence, organicism, and time in composition, and try the same thing out with your own material. Once you understand the concepts involve, you'll realize how many different kinds of music can be made to work with the knowledge of how time plays an integral role in composition. Also, I'd like to point out that these books deal exclusively with tonal music. Tonal music has no monopoly on time, and good contrapuntal skill is always useful even in the most radical kinds of music. If you follow the above plan I can guarantee that wonderful gems of compositional insight are awaiting you, but it will come at a price, and I don't mean the price of the books. That price is time! It will take years to digest all of this (5 years if you're fast, more like 10+ if you're normal like me), but anything really worth doing always takes time. True understanding takes time, there's just no way around it. Of course, a genius composer would work all this out on their own, and wouldn't need a course of study like this. They would simply devise their own because they have an innate sense of what the right questions are when doing an analysis. This course of study is for the rest of us, who need guidance to formulate the right questions that provide proper insight. Reading books is not a substitute for composing, but this course of study is the other side of the coin (developing a fine-tuned musical intellect) to get you asking the right questions, which if you are disciplined, will aid your forward progress as a composer tremendously. Also, you'll be doing some of the same exercises the great composers did when learning with Fux and C.P.E. Bach. The real goal is to become a better composer, not just simply learn about form.