Twitter tends to work viral as well. Our first chapters were linked on Hackernews and Reddit, after one day of tweets and retweets. We just write a tweet when we have finished an interesting chapter or section. Even if the book is not in the open, I think this strategy could work – write a blog post with useful material and tweet about it.
For our book, we also try to create enthusiasm within the natural language processing community, by letting people know about it. ‘Word by mouth’ PR works well :).
Also, getting linked helps. The previous book I wrote years ago (Slackware Linux Basics) had 5000-6000 unique visitors per month by virtue of links and Google searches.
