Wish it would also sync the non-Amazon books, though. Maybe I am not proficient enough yet, but so far I had to copy ebooks into obscure directories by hand (especially interesting on the Android Kindle reader).
Or setup a spideroak sync folder for the ebook-folder on your android, and just sync your non-amazon books through spideroak.
It never works on my mobile for some reason.
It's not perfect, but it's very good and much nicer than doing it manually. Especially for managing collections. Regularly updated too.