Some people may disagree with me, but I still think Asterisk: The Future Of Telephony is still a good thing to read - It’s out of date now (so some of the programming examples in it no longer work) but it will give you a good idea of the scope of what the platform (Asterisk) can do - for FreePBX, the WiKi and the Community (here) are the best possible resource - FreePBX changes FAST and new things are constantly being added/fixed - Dead Trees will never keep up with the pace of change, but the Forums and the Wiki will - Use them.
Here is a link to the book I mentioned: Asterisk™: The Future of Telephony (openbsd.org)
Like I say, it’s out of date, but it’s worth a few nights reading to really understand how the innards work - Back when I started, I was struggling with getting PRI’s (T1 Digital Circuits) working with our Asterisk boxes - this book finally helped me understand it.