Yes, you can absolutely apply for a U.S. Passport Card even if you already possess a valid U.S. Passport Book. In fact, if you already have the book, the process is often simpler and cheaper. If your current passport book was issued when you were 16 or older and is still in your possession (and not expired for more than 5 years), you can apply for the card by mail using Form DS-82. This allows you to skip the $35 "execution fee" required for in-person applications at a post office. The passport card costs $30 and is a convenient, wallet-sized alternative for land and sea travel to Canada, Mexico, the Caribbean, and Bermuda. It cannot be used for international air travel, but many people get it as a backup ID or for "Real ID" compliance for domestic flights. You will keep your passport book while the card is processed, as they are considered two separate documents that can be held simultaneously.