Game books are books that, in addition to being read, are played like a game. This can include books with branching plot lines, book with special game rules, activity books, mystery books, hidden picture books, and various similar genres. Most game books are designed to be solitary endeavors for a single reader. Game books date back to the 1930s and peaked in America in the early 1980s, however, they were mostly killed off by the growing video game market. However, even to this day, they are still being made. I really liked game books as a child, and, to some extent, I still do.
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 and Find Your Fate. Some go a step further and incorporate more complicated game rules as well like the Lone Wolf series. I owned several of these growing up and read about a dozen more from my school's library. I preferred those with coherent stories and complex rule systems rather than those with dozens of completely unrelated endings.
These include books with assorted games and activities in them like crossword puzzles, cryptograms, cutouts, and more. I loved these in elementary school.
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 Picture games from Highlights magazine. I still enjoy these today.