From the site:
Leanpub lets you publish your book while it’s in-progress, so you can get meaningful feedback from your readers and create buzz among early adopters. The people who read your book while it’s still in-progress are the innovators who won’t need to read it by the time it gets to print.
Leanpub lets you sell books on niche topics to people who share your needs and interests, even if they don’t represent a gigantic audience.
The site is free for authors, publishers and causes to sign up and use. We pay authors or publishers 90% royalties minus 50 cents per sale. Also: Leanpub empowers authors to support their favorite causes directly, by automatically sharing their royalties via our Leanpub for Causes program.