Looking for AEL comprehensive Book

I am trying to learn Asterisk Extension Language (AEL). I have read Asterisk Extension Language v.2 from VoIP Info site. I wondered if anyone out here knows a better book on how to use / configure extensions.ael using AEL. Unfortunately, the Asterisk: The Future of Telephony, 2nd Edition e-book doesn’t seem to talk about AEL, except a brief mention on it.