Reddit reviews Man, Myth and Magic: The Illustrated Encyclopedia of Mythology, Religion and the Unknown (21 Volume Set)
We found 2 Reddit comments about Man, Myth and Magic: The Illustrated Encyclopedia of Mythology, Religion and the Unknown (21 Volume Set). Here are the top ones, ranked by their Reddit score.
It doesn't exactly fit your description, but the Man, Myth, and Magic series is an amazing encyclopedia from the 70s. The edition I have fits your physical description, and it was compiled from a magazine of the same name. It has a much more scholarly tone than the usual "booooo spooky" Time/Life books or other pulpy paranormal compilations.
http://www.amazon.com/Man-Myth-Magic-Illustrated-Encyclopedia/dp/185435731X
http://www.amazon.com/Man-Myth-Magic-Illustrated-Encyclopedia/dp/185435731X ?
You may be able to ask the library for their record of what your mom checked out all that time ago. It's likely that they have switched record systems to digital and discarded the old ones or retained them somehow (perhaps you should ask a supervisor...). Otherwise, check the reference section and ask them. They may be able to show you where such a book can be. At least you have a location to start searching.