1. String Theory, vol. 1: An Introduction to the Bosonic String;Polchinski,2007
2. String Theory, vol. 2: Superstring Theory and Beyond;Polchinski,2007
3. Quantum Gravity;Rovelli,2004
4. Covariant Loop Quantum Gravity: An Elementary Introduction to Quantum Gravity and Spinfoam Theory;Rovelli,2014
5. Emergence of a 4-D world from causal quantum gravity;Ambjorn;Phys. Rev. Lett.,2004