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I'm currently reading "Classical Form" by William E.Caplin. 

I'm sure you can find a pdf online.

As much as I love the idea of a textbook that delves into form, I'd recommend score study over theoretical texts. Let me expand:

1. Forms can be overcomplicated and muddied down by extra-motivic/thematic practices (harmonic, rhythmic, and textural). For instance, when studying sonata-allegro forms (from late baroque onwards), one can see the differences each musical epoch had on what constituted 'sonata-allegro' form. While theoretical works on form often overlook or gloss over the differences between musical eras.

2. Forms are often tailored to fit the intra-musical ideas presented within each and every work itself. Forms are nothing more than templates thru which you input your ideas that are developed along conventional norms within our craft. Much the same concept as a potter uses molds. Once you put the clay into the mold, you then craft or stylize it to fit your expectations -tailoring it to fit your tastes and desires.

With these two points in mind, there are some basic formal ideas:

Sectional Forms: Sectional forms are based around specific thematic/motivic elements. You have different types to this. Binary is simply A and B sections. Compound (or Rounded Binary) is simply A and B sections that are followed with a return to the A section to close the work out. This can also be called tertiary form. The key thing to note about sectional forms is that often you want a return to one section -to help combine your ideas more cohesively. This type of form is also one of the oldest. Rondo form is a type of sectional form that is structured either ABACA or a full rondo ABACADABA. There's lots of variety here though. 

Contrapuntal Forms: Contrapuntal forms are based around counterpoint devices and imitation. These types of forms can also be shortened to comprise sections of sectional forms. Fugue, for instance, can appear in a larger sonata-allegro or binary form as an abridged section. Types of contrapuntal forms include fugue, ricercare, and canonic inventions. 

Developmental Forms: Sonata-Allegro as it evolved over the last 400 years is what one would call a developmental form. It holds as its foundation a simple tertiary framework (ABA'). Various epochs ascribed harmonic designations to the sections: A is predominantly in the tonic, B is in the dominant, etc. However, it is the addition in the classical era of a developmental section (AB or C) that laid the foundation of this form for future expansion. Developmental forms serve to develop material formally. Through-composed forms often fall within this category (and some would argue that fugues bridge contrapuntal form and developmental form).

That's pretty much the basics. All musical structures will fall within one of these three form types -with maybe a small handful of hybridized examples. 

As I mentioned in the first sentence, score study is going to teach you more about form then any theoretical textbook will. It will also teach you the most important aspect of form: transitions. Each composer has approached transitions in their own way -so score study will give good insights here.

Hope this helps.

I'm reading Schoenbergs fundamentals of music composition

I agree with @jawoodruff.  This is usually learned in composition lessons.  The one on one study is really to learning this craft.  Quincy Jones is the greatest producer of alltime; he took lessons 🙂

17 hours ago, Left Unexplained said:

I'm reading Schoenbergs fundamentals of music composition

 

Me too, it goes from the simplest structural units of motives to phrases to simple forms like Binary Form(which can be and are often used in larger forms such as rondo form(I mean take for example the Rondo from Beethoven's fourth string quartet, each section is in 2 parts, like binary form.)) and ends with a long and detailed explanation of Sonata Form. So it is like the chapter structure was built around the genesis of a sonata or something.

9 hours ago, caters said:

Me too, it goes from the simplest structural units of motives to phrases to simple forms like Binary Form(which can be and are often used in larger forms such as rondo form(I mean take for example the Rondo from Beethoven's fourth string quartet, each section is in 2 parts, like binary form.)) and ends with a long and detailed explanation of Sonata Form. So it is like the chapter structure was built around the genesis of a sonata or something.

 

Schoenberg is a decent starting point. As I said in my post though, there's a big difference in reading about a form and actually writing in that form. I can boil down any work into its formal constraints -but that's not really going to help you in writing said form. There's lots of variables and important insights that you don't experience when reading about form -not to mention, different theorists often will look at a piece and its construction in different ways (there goes that entire subjective argument so prevalent in art music). 

One good example comes in the work of Mozart. Mozart often used forms in ways different then his peers and predecessors. For instance, he would combine Rondo form with Sonata-Allegro or expand fugal forms by combining them with sectional forms. Large scale works -also- featured an overarching formal structure (the Requiem). Boiling the material down, we also see ingenious use of motivic material within Mozart's works. Do to this, different theorists come to different conclusions when analyzing his music. I've read books that attempt to provide an overarching theory to explain his formal usage and others that go from the microcosmic outward. The interpretation of the structure of his music is oftentimes in the eye of the beholder. 

In lieu of reading a book, I'd look at compositions from the following composers to get a strong understanding of the variability and usage of form:

Brahms, Mozart, Beethoven, Mahler, Stravinsky, Shostakovich, Haydn, Schubert (particularly his liede), Debussy

hi everyone, it’s now popular to read everything online, and I adhere to this trend, I order the best weekly magazines and books in pdf on playing the guitar and re-read in a few hours. I'll look and throw you a good resource 🙄📟🎸