Fabled lands is great, a real masterclass in building a freely traversable world with persistent state using very minimal notation. I love how they wrote little adventures that are triggered in one location in one book, require you to go to a location in another book, complete some task and the return to the starting point, all tacked with a few simple check boxes. Great stuff. One of the authors has a great gaming blog.
http://fabledlands.blogspot.com/