If you're interested in how books and online media can work together successfully, one good example is the "The 39 Clues" project. Created by John Yoo, Rachel Griffiths, and David Levithan, the project is a combination of ten books, an online game, and trading cards.
Here's how the site sets it up: "The Cahills are the most powerful family the world has ever known. 39 Clues hidden around the globe guard the family's power and it's up to YOU to find them..."
The books, cards, and online games all help reveal clues and there are prizes along the way and a grand prize of $10,000. Ten clues are contained in the books, 29 online.
The first book, "The Maze of Bones," was written by Rick Riordan and, the site says, was a number one New York Times bestseller.. The first 12 pages are available on the site. A new book comes out every three months and number nine is coming up shortly as this is written.
Levithan told Fast Company magazine, "This structure isn't going to supplant traditional publishing, but it's going to broaden it. There are things you can do in a multi-dimensional format that you just can't do on 200 printed pieces of paper."
He added, "Kids don't make a distinction between loving the book version of this story or the online game. The perceived gap between readers and gamers are just not that big."
In the first six month they had more than 300,000 registered users, with kids coming back an average of 3 times a week. Scholastic is the company behind it, and DreamWorks has bought the movie rights.
A good model for anybody interested in developing an interactive book/online project--this one is for kids, of course, but I think a similar structure could work for adults.
(For help with any kind of writing, see my book, "Your Writing Coach," published by Nicholas Brealey and available from Amazon and other online and offline retailers.)