I have taken GRE twice. One somewhat mundane comment: please ensure proper rest before taking the exam. The first time I took it, hadn’t slept enough. The environment was so calm and quiet that I fell asleep during the English section of the test – if I remember correctly. Although I woke up before the test had ended, many questions just had to be guessed. Books can be helpful: Princeton Review, Barron’s. However, an online class from, as an example, Princeton Review, can certainly be even more beneficial.