How do I decide which essay to write?

During the exam, the essay titles are out of your hands so all you can do is pick the essay that suits you and your knowledge best and go from there. Some people can be tempted to only learn one book in detail, but I would advise against this, because it seriously restricts the essays you can answer well. Instead, try to go in to the exam with a vague idea of the main themes of the book and therefore the potential essay questions. If you have an idea of what you'd write for each probable essay, then you'll have something to talk about no matter what comes up, and quotes that can be adapted to fit the question.