I tend to have trouble on the SAT Critical Reading section with sentence completion questions, and I was wondering which books or other resources I should use to improve my vocabulary knowledge.
The print resources I recommend for studying SAT vocabulary are Direct Hits Volume 1 and Volume 2 (buy here: http://www.amazon.com/Direct-Hits-Core-Vocabulary-SAT/dp/1936551136/) and Barron's SAT Vocabulary Flash Cards (buy here: http://www.amazon.com/Barrons-SAT-Vocabulary-Flash-Cards/dp/1438070861/). You can also check out this article for links to the best SAT vocabulary lists online: http://blog.prepscholar.com/the-best-sat-vocabulary-lists-on-the-web.
You should also keep in mind that the new version of the SAT (starting March 2016) won't include sentence completion questions. Here are some more details on how to study vocabulary for the new SAT if you won't be taking the test until later next year: http://blog.prepscholar.com/how-to-study-vocabulary-for-the-new-2016-sat.