There are quite a few books on Asterisk out there, but if you can manage Italian I wish to recommend you this one: “Asterisk” (Italian).
I happened to meet in person the author, Diego Gosmar, charing the roundtable on Open Source VoIP at the VON Europe Conference held in Rome in November.
Being the second book on Asterisk written by Diego and his coauthors, it goes pretty much beyond installation and protocols for VoIP, covering in depth topics like how to implement SS7 applications carrier grade with Asterisk or billing with astbill and WildiXbill, but also some spots on ENUM and wireless VoIP.
Mark Spencer, original author of Asterisk, wrote a kind preface for the first book edited by Diego, closing as follows:
It’s especially rewarding to see Asterisk growing in Italy, as I’ve always felt it was long past due time for the America to have something to give back to Italy in exchange for one of Italy’s most important contributions to computer science: the
pizza!
I really hope that the “Italian Open Source Pizza connection” will soon be appreciated also for things like this book 
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