Off topic, but oh man, it’s funny to see the author’s plea to help him combat piracy as compared with the policy of most mathematics authors I’m familiar with. Then again, it is a more “general audience” book.
Okay anyway, if on the off chance you’re interested in actual textbooks on this stuff, the main ones we used in college were Aldwell & Schachter’s Harmony and Voice-Leading and Salzer & Schachter (I just realized it’s the same guy) Counterpoint in Composition. My editorial on them, as, like, someone who cares equally deeply for classical/art music as pop/ular, is that there’s a lot to be learned, but the assumed mindset might be a little tough to fit yourself into. This is, like, notes-on-staff-paper exercise stuff, and both are really more focused on technique and effects than form, per se.
Also in that same vein I’ve been meaning to return to and really give Schoenberg’s Harmonielehre another go now that I’ve clarified to myself what the point of all this theory muck is gonna be for me. Probably a very poor recommendation, it just came to mind.
As for things that focus on form explicitly, there’s really not much that I’m aware of, unfortunately. (NB – I am not an expert.) Part of the reason for this could be (I contend) that there really is no magic to a classical form, just like there’s no magic to the two-verse pop song form. At the end of the day, it’s just a shape to pour your ideas into, and a constraint to be enlivened by.
But, on a very high level, classical forms often involve “subjects” or “ideas” or “motifs,” little prominent snippets of melody that recur throughout the piece. “Hooks,” if you will. The form comes with a few low-level suggestions about what one might do with these hooks, and gives you a few high level “sections” to work within, just as a pop song might be expected to have verses, choruses and that third thing.
Another idea that maybe is only implicit in these books is that classical forms were a… well, a discourse!—a way for composers to be in conversation with one another and their audience. To my mind, some of the most interesting classical pieces are so because of the way they play with the expectations I have of a piece of that type, just as any piece of genre art is perhaps most interesting when it challenges its genre. (Think how Watchmen introduced a note of moral ambiguity into every subsequent superhero piece.)
Sadly, the discourse around many of these forms has really fallen off so much that it’s hard to innovate, just because there are so few audience members able (much less willing) to follow you. Which is maybe why living composers of “art” music typically don’t use them so much?
Sorry for the length and the decidedly impractical “suggestions”