You'll probably know about the difficulty of quantising gravity, and of the arguments over the viability of String Theory as a solution. But what alternatives are there? Well the main contender is Loop Quantum Gravity (LQG). Carlo Rovelli's book Quantum Gravity
gives a research level treatment of this subject. The first part of the book looks at general relativity, classical mechanics and quantum theory, including quantum field theory. However, these chapters don't try to teach the reader these subjects. Rather they provide a new way of looking at them for readers who have already devoted some time to studying them.
The second part of the book gets on to Loop Quantum Gravity itself, showing the importance of background independence - rather than being given, space and time emerge as consequences of the theory. There is a chapter on the applications of the subject, such as quantum cosmology and black hole entropy. The book concludes with appendices looking at the history of LQG and at its philosophical implications. There is no doubt that this book requires a lot of prior knowledge, but I feel that it would be very useful to anyone wishing to learn about LQG, as it has plenty of recommendations of books which will give you this knowledge, and so it maps out a path for those wishing to study the subject.