Karen Armstrong is a very interesting woman — she calls herself a "freelance monotheist." She spent seven yeras as a catholic num, but eventually left, and has taught on a number of topics, including Judaism and Islam.
I read her monograph Islam maybe three years ago — a very interesting, but brief and readable introduction at 200 pages.
This book, A History of God, chronicles the history of the three major monotheistic religions: Judaism, Christianity, and Islam — goes through the similar origins, points of divergence, and parallel trends.
I found the idea of the book super-compelling, but the content was tough for me. Pretty deep and esoteric theology. I’m going to wait for a bit, then try to read her more recent book, The Battle for God — a look at the fundamentalism that has hijacked each of these three religions. Hopefully it’ll be a little bit easier for me to read.