Introduction & Initial setup

Good afternoon all!
I am a long term Linux user as well as a ham radio operator, and recently I came across the HamshackHotline and HamsOverIP. Both of these VOIP services enable configuration of individual endpoints, and also PBX (with the restriction that only one user can access them through the PBX). I have installed asterisk, and started with the configuration, and I wondered if there is a source of documentation recommended to get a better understanding of the VOIP domain.
I see there is an O’Reilly book available about asterisk - would this be a good source? Is it sufficiently up to date, or have there been significant changes recently?
The documentation for asterisk looks good once a basic understanding is achieved.
Alternatively, are there examples of full configurations available to browse through (as opposed to snippets)? I can find examples across the internet, but these are often very outdated.
Many thanks to the developers of the asterisk - it is an impressive piece of work!