Form in Tonal Music by Douglass M. Green & Sonata Forms by Charles Rosen

Both the above texts deal with traditional form for the most part. The Green is a university text, but used copies are always turning up here or there, as it’s a heavily assigned book that results in a lot of resales. The Rosen book helps to put a lot of Green’s information into context. His book “The Classical Style” is also worth a read.