VoIP encompasses a lot of different camps:
H.323 is the most widely deployed VoIP technology.
IPDC/SGCP/MGCP/H.248- All based on the premise of dumb endpoints and intelligent call control. IPDC and SGCP merged and became MGCP which is an IETF draft. H.248 is the ITU variant of MGCP also known as Megacop
SIP- IETF draft based on intelligent endpoints.
You can go to http://www.ietf.org to read up on SIP and MGCP. If you have access to ITU documents read H.323, H.225, H.245 for H.323 based networks. H.248 + the annexes for Megacop networks.
There are many good books out there if you are just starting out Cisco has one called introduction to VoIP.