A History of God
One thing that this book reinforces in my mind is the realization of how philosophically advanced the pre-moderns were. We tend to think of them as primitive, superstitious morons who were arguing about how many angels can dance on the head of a pin, or coming up with easily overturned proofs of God's existence, but that is not the case at all, at least not in the case of the Greek Christians and the medieval Muslims. And in any case, even if their answers are not satisfactory, the questions that they raised are quite sophisticated. I also have a much better understanding of the via negative, the apophatic methods of the Greeks and the Muslims, later adopted by the Rambam and Rabbenu Bachya.