Kingdom of God
I've read it twice now. I highly recommend to anyone who is interested in exploring the Kingdom of God, John Bright's book. It is amazingly entertaining to read, tremendously educational, and wonderfully edifying.
The other two books I really benefited from on this topic are more academic, but still recommendable: Ridderbos and Vos. If you must read something on the Kingdom this very moment, try Riddlebarger.