It seems that functional analysis is greatly motivated by quantum mechanics, right? Is there any reference on the relation between the two?
Read Jean Dieudonné's History of Functional Analysis. More precisely, read §VII.4: The spectral theory of von Neumann. Chapter I of von Neumann's Mathematical foundations of Quantum Mechanics is very interesting too.
I also recommend Dieudonne (following Jose Carlos Santos) for a brief historical account, and also von Neumann's own classic book Mathematical foundations of quantum mechanics. And volumes by Reed and Simon, Methods of Mathematical Physics., if you want some later development. The influence was (and is) very strong, in particular the theory of unbounded operators was developed mostly to satisfy the demands of quantum mechanics.