I'd think it'd be in Psychology.. The University of California at Berkeley seem to have most of theirs in the Education-Psychology department. I can't find where the Los Angeles public library would have theirs in the stacks. Charlotte, NC's online catalog isn't responding. And I can't read the french libraries websites.
So I'm going to say Psychology.
Edit: More information, now that I can access the local library.
Most of the hypnosis books there have a call number of 154. I finally realized that's the dewey decimal system. So that number falls under "Subconscious & altered states".
New Age Hypnosis is listed as 131, which is Occult methods for achieving well-being. (ya know, this is pretty cool.. wonder what other books my library has...)