This is not a book for the casual Sondheim fan. Be warned, unless you have a fair degree of musical sophistication -- and this involves music theory and musicology -- this is not the book for you. But for those who do understand such arcane things as harmonic (including Schenkerian) analysis, or how a typical Broadway tune is constructed, or, to use Banfield's favorite term, the diegetic uses of music, this 1993 book is a gold mine. The emphasis throughout is on the music, not on dramatic construction or the lyrics themselves; they are noted but not dwelt on.
Banfield gives us some historical and biographical background, with a fair discussion of Sondheim's pre-Broadway career (including some tidbits about the collaboration with Bernstein on 'West Side Story'), an overview of the compositional process, detailed exegesis of each of Sondheim's works from 'A Funny Thing Happened on the Way to the Forum' through 'Into the Woods.' There are many scholarly references and a voluminous bibliography as well as a slightly light-weight index. One is disappointed that the book stops in the early 1990s. Perhaps Banfield will update it one day.
This is definitely a book for serious students of Sondheim's work. It is a densely written text but definitely repays close study.