Yes, you can let the U.S. Passport Card expire and just renew U.S. Passport Book if you feel that the former has out lived its usefulness. They are completely separate things so you can just the card lapse and keep it and send the book for renewal.Like you, I probably will have to replace the book before its expiration date as I'm close to running out of pages and I'm also letting the card lapse as I no longer see a use for it.