The Transparent Telephony series is written for technical people who would like to learn about modern telephony. The series walks you through building your own Asterisk PBX system, as well as developing your own code, and much more.
Transparent Telephony - Part 1 - An Introduction
Transparent Telephony - Part 2 - Installing Asterisk
Transparent Telephony - Part 3 - Making and Receiving Calls Using VoIP