In November 2024 we ordered Passport Cards for the first time. The cards arrived 18 days after we mailed in the forms, without paying for expedited service nor even for express mail.
The Passport Card is a stiff plastic card, slightly thicker than our driver's licenses. It is specifically not valid for international air travel, though it can be used to board a domestic flight and is Real-ID compliant. It cost $30, versus $130 for a Passport Book, at least as of the timeframe we ordered in late 2024.
Most importantly though: it is much more reasonable to have a Passport Card with you at all times than it would be to carry around a Passport Book, and the card is a valid proof of citizenship. It can be used within the United States, can be used for travel within the Americas, and will allow re-entry into the US even if you have lost the regular Passport Book.
One caution: when it comes time to renew, both the Passport Book and Card will need to be turned in for renewal. Keep good care of both, if one is lost then the renewal of the other becomes a Lost Passport event which requires DS-64 and DS-11 forms to replace. The DS-11 requires birth certificates and other proof of citizenship, just like getting the passport for the first time required.
A Passport Card is a good way to have proof of citizenship with you at all times.