Gr 5 UpBelieve It or Not. . . is a collection of 40 ``facts,'' some using words such as ``might'' and ``probably,'' classing them as speculations. Each fact/speculation is placed on one page with (usually) a one-paragraph expansion and one or two illustrations. Readers would be better served if the facts were integrated into a discussion rather than sprinkled around this ``Gee Whiz'' format. In contrast, the other two titles are well done. Exploring Mars describes all that is known about Mars from the earliest telescopic observations through the Mariner landings. It details plans for a manned landing and projections of possible findings. Factories in Space tells about space stations; Skylab; spacelab used aboard the Space Shuttle; and NASA's newest space station, to be assembled in the 1990s. This book is an accurate look at these ideas and will be enjoyed by young readers interested in the space program. The three books are attractively illustrated with color paintings and photographs. Margaret H. Hagel, Norfolk Pub . Lib . System, Va.