Should I get a passport book or card?
See my previous post about when you need a Valid Passport.
In some cases, you an use a passport card instead of a book. The cost of a valid Passport Card versus the cost of a valid Passport Book makes looking at this option necessary.
The real consideration is the differences between a standard U.S. Passport Book and the U.S. Passport Card. There are two differences, first and most important “The passport card cannot be used for international air travel.” The second difference is cost, the Passport Card is much cheaper than the Book. The Passport Card is less than half the cost of the Book. Also, remember that both are good for 10 years for U.S. citizens 16 and older but only 5 years for applicants under 16. So if you are getting a passport for a minor you may want to consider the card. All the information on both forms of U.S. Passport are available here:
https://travel.state.gov/content/passports/en/passports/information/card.html