Another tip: A passport card is considered as valid as a passport book within the US, but it is much cheaper to obtain. It is conclusive proof of citizenship and an authoritative RealID-compliant identity card, just like a passport book. Passport cards are valid for land and sea travel within North America and the Caribbean. They're not valid for international air travel (but they can be used for domestic flights).
Edit: Upgrading from a passport card to a passport book is considered a renewal rather than a new application. You can still keep the card if you later want to "renew" it for a book.