Choose Your Own Adventure Books
Two fingers in the book, in case it went badly.
Fighting Fantasy and its imitators: gamebooks where you rolled dice, kept score, and cheated constantly.
The Warlock of Firetop Mountain started it in Britain and the series ran to dozens of titles. You needed two dice, a pencil and no self-control.
Everyone kept a finger in the last page they were happy with. Nobody has ever played one honestly.
Cost back then
£1.50
Goes for now
£5–£40 for first editions