You can use slick wifi with Gutenberg books!

There is a “send to kindle” function which allows you to either use a desktop application, or set up an email address you can send .mobi files to!

Highly recommended if you get tired of plugging in - thats why I swapped from my old Kobo to a Kindle.

Not only that, but this actually adds the book to one's personal library (as "document"). For .mobi, this also synchronizes the last read position, just like it does for books from the Kindle store.