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 (buy here: Direct Hits Volume 1 and Volume 2 and Barron's SAT Vocabulary Flash Cards. You can also check out this article for links to the best SAT vocabulary lists online: https://blog.prepscholar.com/the-best-sat-vocabulary-lists-on-the-web.
You should also keep in mind that the SAT doesn't include sentence completion questions anymore. Here are some more details on how to study vocabulary for the SAT if you won't be taking the test until later next year: https://blog.prepscholar.com/how-to-study-vocabulary-for-the-new-2016-sat.