Introduction to Hilbert space and the theory of spectral multiplicity by Paul R Halmos (Author) Chelsea 1957 2nd ED of 1951 Textbook (9.25x6.125x0.4 in Hardcover NO DJ 114pgs plus ads)
It has been several years since I've read this book, but I have occasionally returned to it for reference. It is eminently readable, even for someone lacking basically any subject-area knowledge. However, it suffers for the same reason, in that it does not get to any majorly interesting results. It serves exactly as the title says, and no more.