This is probably more of a question about what would make a good book rather than a question about this site. However you should aim to include some of the following:
- Something about the history (I suggest you have a look at some of the posts on Jeff's blog)
- The current community and a look at some of the more popular posters
- Hilarious anecdotes taken from answers/comments/chat (I suggest you scour the meta site for those)
- Memorable events (the great rep recalc, unicornify, many memes of meta)
- The future and area 51
- Pepper it (or salt it) with a photo of Jeff and Joel and their own thoughts on how it all went and where it's going
To add some drama you could contact some of the people who have been asked politely to refrain from using the site to add their 2 cents on what went wrong and their future plans.
Put it all together and you should have something worth selling. Good luck!