At a minimum, your media kit should have your biography, high-resolution (300 dpi) pictures of both you and the book cover(s), a synopsis of the book, any press releases you may have, and contact information for you.
Make sure everything in your media kit/press release is professional, polished, and the very best you have to offer. Don?t send/post anything, ever, that you wouldn?t want to see in print later.