Direct Hits has a few vocabulary books that are available for pre-order, but I can't get them all. Which one(s) would you recommend that are best for improving vocabulary and comprehension?
Your options for the 2016 editions of the Direct Hits books are:
If you can only get one of these books, Core Vocabulary is your best bet. It has a manageable number of frequently used words for you to master, so you'll get the most out of your study time. Essential Vocabulary is slightly lower-level (early high school and PSAT words), and Advanced Vocabulary may be overkill based on how much vocabulary knowledge has been deemphasized on the new SAT.