A excellent, easy-to-read book that explains the basics of clear, coherent writing. It's mostly geared towards middle and high schoolers, but as a college student, it did me well when I wanted to refresh my memory. It's not written in a loaded language, either; in fact, it was fun enough to finish for the Goodreads. Highly recommended.
I also recommend the "Painless Grammar" book, which does a perfect job at clearing up all the little grammar kinks that I notice far too many people of all ages struggle with.