What are the best materials to start with reinforcement learning?

Reinforcement Learning: An Introduction by Sutton & Barto

You can also find playlists of lectures from Deepmind/UCL - https://www.youtube.com/playlist?list=PLqYmG7hTraZDM-OYHWgPe... (This is one version of it from 2015 taught by David Silver but you can also find more recent ones on youtube)