I thought about digging deeper into quantum mechanics to make it more intuitive for me and "figure out the principle". I know university level QM very well. Maybe I should gather more knowledge about relativistic QM though. What further concepts are at the core of QM (complicating theories, tensor stuff, ...)? Which books are recommended? I do not wish to learn about all possible approximations and theoretical tools to solve problems, even though only they might make understanding of some phenomena possible. So, only concepts free from approximations. Also I'd try to exclude thinking about particle physics or gravitational effects. This one "approximation" I can accept.