Baron's essential words for the GRE helped me a lot with vocab. I also had a short little book called acing the GRE writing assessment that broke down what they're looking for in essays that I was completing clueless about. Those were both in prep for the old version of the GRE though, so you might want to see if they're still relevant, I haven't followed what they changed in the new GRE. Other than those I just used Kaplan workbooks, which were good but nothing special in my opinion.
