book recommendation for reinforcement learning

Question

I want to start a career in reinforcement learning, I think math is very important in RL, recently I found Sutton and Barrto book, but noticed that this book is very introductory, and doesn't go deep into the subject.Then I found Szepesvari's book, this is very short and I can't use it for research.SO can you suggest some good books for doing research in RL?