If you haven't already signed up for Medicare prescription drug coverage, you have a chance each year during the fall Annual Enrollment Period (AEP)/Open Enrollment Period (OEP). See What is Medicare Prescription Drug Coverage
for more information
"Medicare & You" explains the Medicare Program. It isn't a legal document. The official Medicare Program provisions are contained in the relevant laws, regulations, and rulings.
(Primary Source: Centers for Medicare and Medicaid Services - Medicare and You Handbook. This content may have been enhanced by Q1Group LLC to include further examples, explanations, and links.)