CPT codes are the numbers a doctor puts on an insurance form to describe the services he or she performed. They are published annually by the American Medical Association in several editions of a book known now as CPT 20xx (AMA Bookstore), formerly Current Procedural Terminology: CPT and Physician's Current Procedural Terminology. The book is commonly available in medical libraries.
In addition, CPT Codes are available free for non-commercial use through the AMA's CPT page.