I primarily used the eight Manhattan study guides and found them very helpful, particularly because they focus on math (my weak point) and they provide six practice tests along the books. I would definitely recommend Manhattan. As for the essays, I'd recommend writing practice essays under timed conditions using some of the topics in the topic pool on the GRE website. If you have less time to prepare, what I found particularly helpful was looking at a some of the topics and practicing coming up with a quick outline of how you would respond (the "thinking of an interesting argument/examples" component of the essay is much trickier than actually writing the thing out, I found). If you have any more questions about preparing for the exam, feel free to private message me.