If you're a fan of Jen Wilkin, her three books are currently on sale at $2.99:
I particularly recommend None Like Him. In it she considers ten of God’s incommunicable attributes (ways in which he’s not like us) and shows why that is a good thing. She also shows how we tend to usurp God’s uniqueness by trying to do what only he can do. It’s a powerful combination: deep theology, devotional reflection and conscience-pricking application. The result is the most practical book of theology I’ve ever read.
(Her books are aimed at women, but there's no reason men can't benefit from them too. I do!)