Most US medical schools and many non-US schools require applicants to submit MCAT scores. The MCAT, or Medical College Admission Test, consists of sections on Verbal Reasoning, Physical Sciences, Writing Sample, and Biological Sciences. The MCAT can be intimidating, so you might consider purchasing a study guide or review text. The Kaplan and Princeton Review MCAT guides are excellent choices.
Even if you aren't taking a review course, the Kaplan MCAT Review is an excellent study guide. The Kaplan Review clearly defines the important concepts used in the test. The Kaplan Review is also the easiest review material to locate, so less expensive used books are sometimes available.
The Princeton MCAT Review is another clear and concise study guide. Personally, I think the Kaplan and Princeton Review are comparable, so base a choice between the two on cost or convenience. If only one or the other is available, rest assured you are in good hands. My final advice? Don't Procrastinate. If you aren't procrastinating, then RELAX!
In addition to the MCAT Student Manual and the review courses, there are numerous publications covering medical college selection, admissions, and financial aid. The MCAT Student Manual is available at the MCAT web site and accompanies your registration materials. You can find additional resources on-line, at campus bookstores, and at used bookstores.