[*] SIP for Dummies

[Bill Reid]

Not a bad downloadable PDF. You need to register(free) but there are many other 
tutorials that you may find interesting.

Abstract: SIP For Dummies
Avaya Limited Edition

This Avaya limited edition "SIP for Dummies" Book describes SIP from both
business and technical perspectives. Read about SIP architecture and
operations, its impact on business, discover how SIP can improve
communications, how SIP technology works and how to build a SIP
environment.

Registered users at Webtorials may access the information directly at:
http://www.webtorials.com/main/gold/avaya/index.htm
and others may access the information after registering at
http://www.webtorials.com/register.html