So, I was looking at picking up American Sphinx, but I've seen a lot of people say that it assumes you already know a great deal about him. What is the definitive biography (preferably one available on kindle) of him?
Not sure how accurate it is with the current state of Jefferson scholarship, but a classic biography of Jefferson is Dumas Malone's 6 part, pulitzer-winning biography Jefferson and His Time.
That said, it is a bit of a bear given its length and given its age I'm not sure how well it has stood the test of time. Someone more knowledgeable than me might be able to give you a better assessment of its current reputation, or a recommendation for a more manageable (and recent) single volume work.
I liked Hitchens' Thomas Jefferson: Author of America. Ari Helo's Thomas Jefferson's Ethics and the Politics of Human Progress: The Morality of a Slaveholder is also pretty good, as is M. Andrew Holowchak's Thomas Jefferson: Uncovering his Unique Philosophy and Vision. Annette Gordon Reed also has some of the best books about Jefferson specifically regarding his relationship with Sally Hemings.
Avoid anything by David Barton or Paul Finkelman; they're highly biased in their characterizations of Jefferson. Also avoid Henry Wiencek for the same reason.