I don't like kindle free ebooks. I read their version of pride and prejudice and anna karenina but there is no TOC in it. And of course it too can not be
easily converted to another format like epub. Google books has so much typo and inconsistent. So my main source of free ebooks is
http://feedbooks.com and of course here in MobileRead uploads.
As far as i know there is only epub format in feedbooks, but it's really easy to convert it to another format using calibre (And I like feedbooks layout and consistencies)