Call of the Minaret by Kenneth Cragg
I thought I'd post a few book recommendations. Cragg is a former Anglican bishop of Jerusalem and is one of the great scholars of Islam of the previous century. His prose is dense and challenging, and he may seem to treat Islam with kid gloves, but if you learn how to read his subdued style he offers some poignant and crucial criticisms of Islam. Also, his focus on approaching Islam from a Christian point of view is helpful. FInally, this book was written before 9/11 which means it still has a sort of innocence to it, and can thus avoid being an apologetic for Islam or a screed of denunciation.
Available at Amazon, among other places.