AsteriskNOW is an ISO that includes both Asterisk, the engine that powers the communication that occurs in the system, as well as FreePBX, a UI/system built on top of Asterisk.
If what you want to learn is how that system works “under the hood”, then Asterisk: The Definitive Guide along with the resources on the Asterisk wiki are a great place to start. A:TDG also includes some good overviews on SIP and telephony.
If what you want to learn is how to use FreePBX, then you should probably head over to their forums. The folks over there can point you in the right direction.