<p>Barron's SAT II Writing guide helped my grammar greatly. In my writing class, we used "The Little Brown Handbook" or something along those lines (the book is not little or brown though). It seemed to be quite comprehensive.</p>
<p>Grammar improves when you practice and study. Practice by writing and rewriting.</p>
<p>Well, you don't NEED to know anything difficult grammar-wise for the writing test, but those arcane rules are good to know anyway for college; the little brown handbook, like somebody else mentioned, is very comprehensive.</p>