lucy wrote:If you want to write in a classical style, I'd also recommend Schoenberg 's Fundamentals of Musical Composition. It is a classic.
I know someone who has studied the book and it discusses how pieces are constructed, from the smallest building blocks to a complete work. As far as I understand all pieces consist of a few fundamental units and these are varied upon and developed to create a coherent complete work.
Schoenberg uses the Beethoven piano sonatas extensively to illustrate his argument.
You could also consider enrolling on the Open College of Arts' composition courses. The person that read the Schoenberg book is studying with them and IMHO his compositions have improved greatly. Fantastic improvement, in fact!
The Schoenberg is a good book, but the language can be quite troublesome. Schoenberg invents his own terminology for things we have since standardised, and his explanations aren't always the clearest. Also, despite its title, it only really focuses on structure, and leaves off voice leading, harmony, etc. That said, it is a great guide to structure.