I haven't read the book, but if it's a novel or short story, coming up with a thesis shouldn't be that difficult. Just ask yourself "what is the author trying to say in this story." Your answer will either be a suitable thesis statement or something that can be made into one.
What exactly do you want to prove? Do you want to analyze the meaning of the book? Do you want to analyze the characters? Do you want to relate the book to real life? You need focus before you write a thesis.
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.