Every great open source project needs great documentation. Check out the Asterisk Administrator's Guide for a complete overview of configuring and operating Asterisk. See the Asterisk Reference Guide for help with Dialplan applications and functions, CLI, AMI and AGI commands and in-depth info on Asterisk configuration files.
Asterisk Documentation
Command Reference
Dialplan Applications
Dialplan Functions
Asterisk Resources
Get Started with Asterisk:
Build Applications with Asterisk:
Advanced Topics with Asterisk: