Every time you try to preview or publish a new version of your book, we save a version of your manuscript on the Manuscript Versions page here:
https://leanpub.com/YOUR_BOOK/versions
...making sure to replace YOUR_BOOK with your book's unique web address.
On the Manuscript Versions page, you will see a record of your actions, like this:
For the version of your manuscript you would like to download, click the "create archive" link under "Export" to the right.
Note that sometimes the "Last Edit on Version" is the same time. This means that you created new versions (by, say, previewing or publishing) but the book manuscript content was unchanged.
When the process completes, you will see an ugly page that looks something like this:
Click the lonely blue link that says "download". This will give you a .zip file containing a copy of your manuscript folder for that version.
You will now find the zipped manuscript file on your device, wherever you keep your downloads:
After you've completed the download, double-click the file to unzip it. In the unzipped folder, you will see your manuscript files:
The next time you go back to the Manuscript Versions page, under "Export" to the right, you will now see that you can download the manuscript files again any time:
Note that if two versions correspond to the same manuscript version, all the download links will download the same zipped manuscript. So, in the above screenshot, all four download links above will download the same thing.