Biggest gains are by PowerScore CR and Kaplan Verbal Workbook. Official Guide is the leader as usual. This month, the new OG 12 came out, so the references really shot up:
Rank Previous Rank GMAT Club's Top 20 GMAT Books
1 (1) The Official Guide for GMAT Review
2 (2)
Manhattan GMAT Sentence Correction
3 (3) GMAT 800
4 (6) Kaplan Verbal Workbook
5 (10)
Manhattan GMAT CR Book
6 (11) Powerscore Critical Reasoning Bible
7 (9) Kaplan Premier Program
8 (7) Kaplan Math Workbook
9 (16) GMAT for Dummies
10 (4) Official Guide for Verbal Review
11 (8) Cracking the GMAT by Princeton Review
12 (12)
Manhattan GMAT Number Properties Guide
13 (13) Project GMAT Advanced Math
14 (14)
Manhattan GMAT Word Translations Guide
15 (15) Powerscore Sentence Correction
16 (17)
Manhattan GMAT Geometry Guide
17 (20) The Ultimate Math Refresher for the GRE, GMAT, and SAT
18 (5) Official Guide for Quantitative Review
19 (18)
Manhattan GMAT Fractions, Decimals, and Percents Guide
20 (19)
Manhattan GMAT Equations Inequalities and Vics Guide