Mysteries of the Unknown is one of the better-known series by Time-Life. It was heavily advertised back in the late 80's and early 90's with distinctive, long commercials. Each volume is hardcover, black, and has a colorful illustration on the cover.
I have seen many of these volumes at used book stores and book fairs, but I haven't completed my set yet (I have 15). Just flipping through them, though, they seem to be very interesting and well written.
There have been several re-printings of these volumes, including a Spanish version. The originals were all printed before 1993, though, so you can probably tell by the date whether or not your book is an original.