https://mongoliad.com/
What is the Mongoliad? It is a serialized book, web site, online community. It's hard to describe exactly. Neal Stephenson, Greg Bear and other authors are collaborating on a medieval book/world/experience.
The website has been up for awhile now, but I waited until the iPad app came out -- which happened this week. It's a very interesting experiment into what a book could mean in the age of technology. You can see the book come together. Talk with and give feedback to the authors and help SHAPE the direction the book takes. Eric Flint did something similar with his 1632 book after it was released.
You can sign up for free, and there are subscriptions ($5.99 for 5 months, $9.99/year) but I'm not quite sure what extra you get for subscribing.
By the time I came upon 1632, the community had been going a long time. This is a chance to get involved early and see/shape how things develop.
Lee