I finished reading "Linear Operators for QM" by Jordan a couple of weeks ago, and it was extremely interesting for me. It introduced various topics in functional analysis that are relevant to the foundations of QM (Hilbert spaces, spectral theory, von Neumann algebras, etc.). I was wondering if anyone could recommend some pure maths books that go into these topics in more detail, or any other physics books that explain the mathematical foundations of QM written for advanced undergraduate/early graduate students. If you know about any books on an introduction to functional analysis in general, then that would be great too. Also, how much measure theory and Lebesgue theory would I need? My only exposure to those things are from Rudin's mathematical analysis and the QM book I've mentioned.