Other Resources
As mentioned in the landing page for this primer, it's not nor was meant to be exhaustive.
Here are some other resources for information.
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 Freenode, the #asterisk channel is a good place to ask questions and get help from live people. I go by ChannelZ there, if you care.