I have a passport card for driving into Canada, which I do regularly. Now I am planning a Caribbean cruise departing from and returning to Miami. Do I need to have a passport book or will the passport card suffice?

What a great question! The best answer to cover all bases is to have both. Under normal circumstances it is a faster process to use the passport card to cross the US/Canadian border. But think about this: What if you are cruising the Caribbean, become ill, and need to be evacuated to land and flown home? Passport cards are not valid for air travel so in order to fly home you will need to have a passport book, and obtaining one in an emergency may be very difficult and add additional stress to an already stressful situation. Better to have both the card and the book so you are covered for any situation.