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 book.
If you can only get one book, get the SC book from
MGMAT. Good luck!
I hope that helps