Some (such as bb) consider Nova a decent product, though the majority of test takers do just fine with other guidebooks. For me, there isn't enough recommendations for Nova.
If you want practice material, then go for the GMATClub tests
and Sackmann's Extreme Challenge set (as georgepaul0071987 suggested). At your level, purchasing another guidebook just for practice is not advised. Continue to use whatever books you have. Rely on an error log
and review the pertinent sections.
To go from your current level to 51, a great deal relies on fine-tuning and timing. Also, observe your stamina level toward the end of the math section.
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