Game books are books that, in addition to being read, are played like a game. Game books can include activity books, books with branching plot lines, hidden picture books, and various other genres. The were extremely popular in America in the early 1980s and I liked them a lot, however, they were mostly killed off by the growing video game market.
These include books with assorted games and activities in them like crossword puzzles, word finds, cryptograms, coloring pictures, cutouts, and more. I loved these in elementary school.
Branching Plot Books
Branching plot books are stories where the reader is allowed to choose, from a limited number of options, how the plot will progress. Books like this include Choose Your Own Adventure, Lone Wolf, Find Your Fate, and many others. I owned about a dozen of these growing up and read more from my school's library.
Hidden Picture Books
These are books with collections of drawings in which pictures are hidden in them. Popular titles include Where's Waldo, For Eagle Eyes Only, and compilations of the Hidden Pictures in Highlights. I still enjoy these today.