In the books the reader directs the action, choosing where to go next out of a few options. As they say on the cover, “YOU become the hero.” The books were somewhere between a story and a game. You were supposed to use dice too but I think most readers bent the rules plenty. I know I did. Most of the adventures were set in a kind of Middle Earth type world called, if I remember correctly, Titan. Dragons, orcs, hobgoblins, that kind of thing. However science fiction stories were sometimes thrown in too.
It was the illustrations that I was interested in as much as anything. I loved this sea monster, drawn together from discarded bones. Look at the strange fish-with-arms beasts coming out of the shadows either side of it. Cool.
I think the best storyline was Slaves of the Abyss, that book would make a good movie. Some of the game books were themselves based on movies, not in their plots necessarily but in their atmospheres or general feel. Starship Traveller was like Star Trek and Freeway Fighter was like Mad Max. In retrospect I realise The Rings of Kether was based on Bladerunner and the work of Philip K. Dick. This story, number 15, was set in a seedy future, where technology has not saved the human race but rather given us even more ways to be bad.
In The Rings of Kether your mission is to crack an intergalactic drugs cartel. You travel around and sometimes go undercover. There were drug dens inhabited by genetically-adjusted slobs. There were weird mutants, one like a worm with a woman's face and a long tongue. And these books were supposed to be for kids! I recalled that some of the illustrations to The Rings of Kether scared me a bit. But it did not matter, I was hooked.