Forget books. You have the world's largest information database at your fingertips.
http://www.euronet.nl/~rja/Emusic/Synth-diy/
There's a mailinglist for synth DIY. Lots of synth DIY hobbyists and few total geniuses. If you have problem they are glad to help. A word of warning: When starting to build a synth, prepare to be frusturated when nothing works on first try. ?hih.
You find lots of synth DIY schematics from the internet. Just search. I'd recommend doing something based on ASM1 design because it's the most popular so there are lots of people out there to help you out.
By the way It's quite funny that today I decided to start my synth project. I'll be ending up with a semi-modular synth built inside a cheap late 70's psychedelia organ. I'm doing the design at the moment ant it will be (I hope) pretty insane stereo synth.
Feel free to contact me on this synth business. I'm eager to share thoughts. Maybe shared PCB images or even front plate designs.