Strunk, W., Jr. & White, E. B., 1979. The Elements of Style, 3rd ed. New York: Macmillan, 85pp.
Guide to Grammar and Writing from the Capital Community College Foundation. Check out the Essay and Research Paper Level and the quizzes.
Writing Resources: a series of handouts developed by Harvard University's Writing Center that focus on writing essays. Topics include essay structure, thesis development, topic sentences, transitions, conclusions and revisions.