Widely acknowledged as the second ever modern roleplaying game, Tunnels & Trolls was first released in 1975 to be a more simple and accessible game to Dungeons & Dragons. It gathered quite a following in the early days due to its elegant and cut down system. The system suited standard roleplaying games, but also Choose-Your-Own-Adventure book style games as well, and many of these were written.
If you want to replicate the original T&T experience, finding a copy of the 5th edition (or 5.5 if you can) will suit you. This edition was released in 1979 and is probably the ‘classic’ version of the game. More recently, a 7th edition has been published which adds a skill system and ‘modernises’ some of the game mechanics.