I've had a proof based linear algebra course as a freshman, where I learned that the spectrum of an operator was the set of the eigenvalues of that operator. Now in quantum mechanics I learned that this isn't true and that the spectrum of an operator can contain infinitely more numbers. Also in my course I've never learned anything about vector spaces of infinite dimension. I'm getting lost with the linear algebra part of QM. Could you please recommend me some book(s) that deals with linear algebra (better if it's aiming at physicists) with vector spaces of infinite dimension? Thank you very much.