For 51 in quant, I would suggest Jeff Sackmann's 1,800 questions and GMAT Club tests
. I have not tried either (will in a few months when I start studying to retake the GMAT), but both get excellent reviews. Even if you do not reach 51, a slight bump in verbal will send you over the 700 mark.
To do well on verbal, one must master SC. I would purchase the SC book from MGMAT. That is the best book for SC, whether you are a native speaker or not. For CR, take a look at PowerScore CR
. As for RC, I would suggest to start reading fiction books. Have a look at this thread
. I would also suggest working on OG problems from OG 13
and the blue OG verbal
If you can only get one book, get the SC book from MGMAT. Good luck!
I hope that helps
If my post helped you, please consider giving me kudos.