Rule-Based Programming in Interactive Fiction
http://www.eblong.com/zarf/essays/rule-based-if/index.html
cyoa
http://samizdat.cc/cyoa/#/_

As a child of the 80s, the Choose Your Own Adventure books were a fixture of my rainy afternoons. My elementary school library kept a low, fairly unmaintained-looking shelf of them hidden in one of its back corners. Whether this non-marquee placement was an attempt by the librarians to deemphasize the books in favor of ‘serious’ (children’s) literature or was simply my good luck I still haven’t worked out. But it meant there was a place that I could retreat to and dive into unfamiliar worlds without distraction.

Amazing website dedicated to choose your own adventure books

In the story, your concord flight is interrupted when you are beamed aboard a nearby spacecraft trolling the universe for intelligent life. Once aboard you discover your new captors, the U-TY, are interested in keeping you around only to the extent that you can help them find Ultima, the ‘planet of paradise’. The planet’s location is cloaked in mystery and you are only told that it’s a place that cannot be reached ‘by making a choice or following directions’. However this is all foreshadowing for when the reader finally becomes frustrated in the apparently impossible quest and begins flipping through the book hunting for that ending. In fact not choosing is the only way to reach Ultima. The branch diagram for UFO 54-40 is unique in that it has one ending – the Ultima ending – which is completely disconnected from the rest of the story. It exists as an island, unreachable through choices but discoverable thanks to the random access nature of the book.