Asterisk: The Definitive Guide
Often referred to in the #asterisk IRC channel as The Book, available to read online, download as PDF, or for purchase as an actual printed book.
The Asterisk Wiki
The official documentation for Asterisk is available on the Asterisk Wiki.
The Asterisk IRC channel
Hosted on the Libera Chat IRC network, the #asterisk channel is a good place to ask questions and get help from live people. I go by ManChicken there, if you care.