gsm specifications

Hello, everyone! I've got c123-compatible phone, the firmware is running and that's really cool. Testing branch even sometimes says that a call has been released, but it doesn't seem to reach the target (no ringing). I would like to debug and maybe contribute, but... But here is a problem: I don't understand GSM and can't find reasonable documents to read. I've looked through a book on gsm "GSM and Personal Communications Handbook", but it doesn't seem to provide enough info. Specifications of GSM I found at http://www.3gpp.org/specification-numbering are too plentiful. Could you please advise some books to understand the protocols? Or could you tell me where to look for specifications and which should be read first (an what to be read at all) ?