This concise text for advanced undergraduates and graduate students covers eigenvalue problems in classical physics, orthogonal functions and expansions, the Sturm-Liouville theory and linear operators on functions, and linear vector spaces. It emphasizes the unity of a variety of techniques and is enduringly relevant to many physical systems. 1962 edition.
Great supplementary book if you are curious about inner workings of math in quantum mechanics. I have started studying undergrad quantum mechanics before reading this book and matching the concepts in this book to physics was fun! The book does not hold your hand by explaining to you which one of those concepts in quantum mechanics applies here, at least at the beginning and that makes it much more fun to explore in my opinion. Problems are pretty balanced with regards to how difficult they are.