My knowledge of music synthesis comes mostly from these two books:
Deutsch., ed., The psychology of music, 1982.
Chamberlin, Musical applications of microprocessors, 1980.
The Deutsch book has gone through a couple more editions, the most recent being in 2013, but in any case it's only tangentially about music synthesis.
Are there any recent surveys of music synthesis that I can use to update my knowledge? I'm mainly interested in understanding the theory and mathematical techniques. Not particularly interested in reading how-to books that discuss particular proprietary hardware or software.