Wiardgroup

Dave Hylander (maker of the Mini-Wave expander PCB)
has the ARP 2600 users guide online as a .pdf

I highly recommend everyone look this over.

It is a valuable introduction to synthesis in general.

It is also helpful for grasping difficult ideas like
how audio signals can be control voltages when used
in the "low" range.

http://www.hylander.us/images/schematics/arp/Arp2600Manual.pdf