I also found this:
All free books on Feedbooks are available in a Kindle compatible format (for both Kindle devices and mobile applications), but due to the closed nature of the Kindle ecosystem, none of the files available on our store are compatible with the Kindle.
To download a book in a Kindle compatible format, click on the book's name and then select the "Kindle" download option.
Mobile Website
You can also easily browse Feedbooks on your Kindle using our mobile website:
http://m.feedbooks.com