Series of books authored by Steve Jackson and Ian Livingstone (whose entries in the series were, IMHO, vastly superior to SJ's inconsistent efforts), the first one of these, The Warlock of Firetop Mountain was an academic experiment which struck it rich. These gamebooks played in the same manner as choose your own adventure books with the added element of die-rolling and game stats. Fell out of fashion when gaming systems got really big and people stopped reading.