In an impressive and laudable display of foreward thinking and action, the American College of Neuropsychopharmacology has put an entire textbook online that compiles all we currently know about how drugs affect the brain. The tome is a wealth of information, and now it's freely available as PDF files.
Over 131 chapters detail the history and development of a variety of different medications and discuss how their effects on brain physiology, cognition and behavior. Best of all, you don't have to be a neurosurgeon to understand the text.
It's recommended reading for anyone curious about the medications involved in treating brain injury.