Here are some of the best works on Quantum Gravity:
- Bryce S. DeWitt, Quantum Theory of Gravity. I. The Canonical Theory
- Claus Kiefer, Quantum Gravity, International Series of Monographs on Physics
- Carlo Rovelli, Quantum Gravity, Cambridge Monographs on Mathematical Physics
- Steven Carlip, Quantum Gravity in 2+1 Dimensions, Cambridge Monographs on Mathematical Physics