Long-running (in quantity, if not in years) series of "guess-the-ending" mysteries featured a couple of distinguishing characteristics: the cover illustrations included clues that helped the reader solve the mystery, and the final solution was bound together so that it could only be read by cutting part of the book open, preventing accidental peeks. The basic formula was revived years later (aimed at a younger audience) in the You-Solve-It Mysteries series.
