The best book I may have read on religion is
A History of God
The problem with the book is that it is very dense, with a lot more detail about a lot of religions than you might want. But many sections are fascinating. And Armstrong does a nice job of articulating a theory of how conceptions of God have changed according to the needs of the time.
I encourage anyone to read it, with the caveat that there are probably some parts that you will want to breeze through.