From Sonic Retro
A gamebook is a book with a branching storyline that serves as a medium for gameplay.
The reader plays the game by choosing which of the branches of the storyline to follow. This usually occurs at the end of any given page; the reader will then be prompted to choose between one of several possible courses of action for the plot to follow, and to turn to the appropriate page or section. Most gamebooks have multiple possible endings, at least one of which is a "win" with most or all of the rest being "losses"; the added replay value of gamebooks is one of the primary reasons for their popularity.
Most gamebooks are written as though the reader were the protagonist, and are written in second person narration. Some gamebooks, particularly those derived from role-playing games, also require the reader to keep track of the main character's resources or status, upon which certain branches of the storyline may be dependant.
Popular gamebook series include Choose Your Own Adventure, Lone Wolf and Fighting Fantasy.