Oops time Leanpub supports multiple storage engines and a private GitHub repository is probably the safest way to the backing up your working folder. I chose Dropbox as I didn’t envision anything wrong with the automatic synchronization mechanism. While working on my book, I accidentally managed to wipe-out half of my diagrams and all changes were instantly synchronized by Dropbox. The free-of-charge Dropbox account doesn’t offer folder-level versioning, so deleted files are simply gone. Fortunately, IntelliJ IDEA Local History saved the day and the diagrams were properly restored.