The Collector's Library of the Unknown is a set of reprints by various authors. The unifying theme is 'the unknown'; from holistic healing to palmistry, all of these books deal with unsolved mysteries or other unusual circumstances. I haven't read these yet, but they seem pretty interesting.
Physically, these books look a lot like the Mysteries of the Unknown series, and sometimes they can be confused. Be careful!
Some of the authors in this series are noteworthy; Harry Houdini, Upton Sinclair, and Sir Arthur Conan Doyle (Sherlock Holmes, The Lost World) are probably the best known.