Leanpub is a platform for publishing books that gives author control and tools beyond those available from traditional publishers. I used it to test an idea for a Solr Clients book and was able to validate its lack of traction without spending a cent.
One of the great features of Leanpub is that a book can be updated and the readers get notified of the new version. Consequently, it is sometime useful to keep the old versions around for the author and sometimes even for the readers.
This feature is not currently present in Leanpub, but we can add that with a bit of a glue from Zapier - web automation service. This is especially useful if you are using GitHub to edit your book with automatic preview and want a full end-to-end edit-preview experience without having to touch the Leanpub website.