What the world believes
The Universe Next Door
Gives a quick little intro to and critique of lots of basic forms of belief. Very helpful.
Islam in Context
Peter Riddell and Peter Cotterell
'One of the best introductions to Islamic history, and what Muslims believe.'
Faith to Faith
'A really good introductory book on Islam.'
Christianity at the Religious Roundtable: Evangelicalism in Conversation with Hinduism, Buddhism and Islam
Timothy C. Tennent
How should evangelicals respond to world faiths? An excellent read for anyone studying world religions.