For those unfamiliar with the term (and who haven't read any Choose Your Own Adventure books), a gamebook is a book consisting of numbered episodes or chapters which are not to be read in sequence. Rather, there's an instruction at the end of each episode as to which episode to read next. Sometimes the instructions are simple ("Go to 12") while other times the reader is presented with a choice and can affect the flow of the story depending on which episode (s)he chooses to proceed to.
This extremely simple concept of interactivity led to the creation of hundreds of gamebooks of varying complexity. The simplest among them (and I include the CYOA series in their number) have about 100 episodes each and offer an occasional binary choice. One path leads to death or loops back to the main chain while the other advances the story. More complex gamebooks have more elaborate decision trees, typically leading from one large, literary 'key' episode to the next, with alternative ways to reach each subgoal, and are enhanced with RPG elements like item management, Hit Points and more elaborate battle systems.
Incidentally, gamebooks were disproportionately huge in Bulgaria when I was growing up. The early 90s found Bulgaria fresh out of the Eastern block with a collapsing economy and a crushing inflation. This was a perfect breeding ground for gamebooks: teens and young adults were starved for gaming, but personal computers were expensive and hard to come by. So, when Lubomir Nikolov (the man who translated J. R. R. Tolkien's Lord of the Rings series into Bulgarian) stumbled across an American gamebook in a used bookstore in Sofia, he saw the potential of the genre. The first Bulgarian gamebook, titled "In the Labyrinth of Time", was written by him under the penname Colin Walumberry, published by Pleiada in 1992 and kicked off an 8-year gamebook golden age in Bulgaria.
After over a decade of drought, the genre is seeing a resurgence in Bulgaria. In December of 2012, an interactive story I wrote under the penname Peter Vale about a Bulgarian nobleman trying to escape from the Andalusian city of Cordoba as it's being conquered by Ferdinand III of Castile was published in Bulgaria as part of a gamebook collection alongside authors Sycamore Bright, Al Toro and Nol Keldon.