I just finished the Microsoft Exchange Server 2013 High Availability book from Nuno Mota.
I highly recommend it as the definitive resource for HA. But, I also recommend this book as the perfect reference for how many components of Exchange actually work. From Database Availability Groups  to the inner workings of the  Client Access Server.  Although the book is only 266 pages long it’s packed full of great information!
I highly recommend it.