I recommend 'Islam: A Short History' by Karen Armstrong.
Karen Armstrong is not a Muslim. In fact, I believe she is an ex-Catholic. She now calls herself a monotheist, but does not afiliate herself to any of the three monotheistic faiths.
I found her book positive towards Islam and informative, without being pushy or preachy in any way ...
'A History of God' by the same author looks interesting too, but I haven't read it myself.