The thesis is the essay itself. It is the core at the center.
BTW my previous post made it seem like you should have a quote as part of the thesis... that was not what I mean to say!
The thing to do is ponder the book, and whatever your most meaningful insight from reading it happens to be -- that is the thesis.
Capturing the thesis is the art of writing, really. Convey the thesis. In order to convey your own original thought on the content, you need to first really engage the content.
Think of the book that has influenced you the most. I bet it would be easy to come up with a thesis sentence about it. The deeper you go into the book, the easier it is to articulate and back up a thesis.
Kevin, EssayForum.com