When I was a kid, I used to read tons of gamebooks. They're basically fantasy-themed (for the most part, there were a few sci-fi ones too) Choose Your Own Adventure with the addition of some very basic RPG elements: a couple of skills, some dice-based combat. Some of them had systems where you could gain XP, level up and improve your stats and learn new powers. One of my favourite series was Lone Wolf, where you could bring your character from one book to the next.
In the past year or so, I discovered a series by a couple of my favourite gamebook authors that I'd never heard of before called Fabled Lands. It was released in the mid-90s when I was at college in the US (gamebooks never seem to have really caught on here) which is why I missed out on it. It sounded really amazing. Essentially it was a MMORPG in book form: there were going to be 12 books in the series, each representing a region in a huge world and you could freely move from region to region (the book would direct you to another book/section). Gamebook popularity was fading at the time though, and only 6 of the planned 12 books were published.
Gamebooks have been undergoing a bit of a revival in recent years. People who read them as kids are reaching that age where you have disposable income and you get nostalgic for stuff you liked as a kid (like me). I was excited to find out the first four books of Fabled Lands had been reprinted, and the 5th book just came out (6th one should be coming next month). If the next two books do well, there is a possibility that the series will be continued! Fans have suggested doing a Kickstarter (which I would be happy to support) but apparently there is no Kickstarter for UK companies yet.
Here are Amazon links to the books:
1: The War-Torn Kingdom
2: Cities of Gold & Glory
3: Over the Blood-Dark Sea
4: The Plains of Howling Darkness
5: The Court of Hidden Faces
Unrelated to Fabled Lands, one of the co-authors of the series also wrote an interactive fiction iPhone/iPad app based on Frankenstein that tells the story from the monster's perspective. It's immersive and thought-provoking and I'm really enjoying it so far! The Chimerist has a good review of it.