This is a good and short introduction to RL. The density of the information in Spinning Up was just right for me and I think I've referred to it more often than any other resource when actually implementing my own RL algorithms (PPO and SAC).
If I had to recommend a curriculum to a friend I would say:
(1) Spend a few hours on Spinning Up.
(2) If the mathematical notation is intimidating, read Grokking Deep Reinforcement Learning (from Manning), which is slower paced and spends a lot of time explaining the notation itself, rather than just assuming the mathematical notation is self-explanatory as is so often the case. This book has good theoretical explanations and will get you some running code.
(3) Spend a few hours with Spinning Up again. By this point you should be a little comfortable with a few different RL algorithms.
(4) Read Sutton's book, which is "the bible" of reinforcement learning. It's quite approachable, but it would be a bit dry and abstract without some hands-on experience with RL I think.