Recommended: God is not One

Stephen Prothero's book God Is Not One: The Eight Rival Religions That Run the World - and Why It Matters (2010 - see book list on sidebar for details) is a useful introduction to the eight largest religions in the world. In particular, he makes the point that despite ecumenical attempts to argue that all religions are simply different paths to the same God, the eight major religions differ at a fundamental level - they see different problems in the world and seek different solutions to their different problems. Christians, for example, worry about sin and seek salvation, while Buddhists worry about suffering and seek nirvana (many without any god required) and Hindus worry about getting off the wheel of reincarnation.

I appreciate the motives for trying to see commonality among religions, given how bloodily divisive these religious differences have been throughout history, and even to this day. But I do notice that most of those who argue all religions are just different paths to the same god all seem to assume (tacitly, at least) that that god is in fact their own god.

Any book about religion is of course going to be contentious, but I think this is a useful approach to understanding the core of each of these major religions -- what they see as the problem in the world and how they try to address that problem.