DailyMed has listings for over 76000 FDA-approved drug listings for humans and animals. It is the official provider of FDA label information. It includes an archive of labels and a tablet/pill identification tool.
AccessMedicine offers hundreds of drug monographs that you can search by the drug's generic name, trade name, or class. This resource also includes some patient handouts describing common side effects and warnings.
The Medical Letter provides accurate and practical information on drugs and treatments of common diseases without the influence of the pharmaceutical industry.
Goodman and Gilman's the Pharmacological Basis of Therapeutics, 13th Edition by 
Pharmacotherapy Principles and Practice, Fifth Edition by 
