Allen Strange. Electronic Music: Systems, Techniques, and Controls 2nd Edition (1983)
Allen Strange's book is perhaps the definitive text on modular synthesis methods, written from a standpoint that's musically understandable, as opposed to requiring a part-background in electronic engineering to understand. Not that Strange doesn't go into the engineering concepts; he most certainly does, but everything here is treated in such a balanced, even manner that allows those interested in the musical concepts to have their desire for information satiated, and those looking for engineering concepts will find the book equally well-ordered.
Useful both as a textbook and a reference document, the book also contains numerous illustrations and diagrams of modules by Moog, ARP, Buchla et al. I consider this to be an invaluable work for those exploring modular or most any other analog synthesis methods, and I also consider it nigh-criminal that such an important work should be allowed to go out of print. Someone should pick this back up and reissue it!