Thanks. I was finally able to get the renewal PDF form, and I see where you can request the passport card only. However, since you must send in the passport book, I would love to see a statement to the effect that the original passport book will be sent back along with the card.

I suppose this note on the renewal form implies that that is the case:

NOTE: You may receive your newly issued document and your returned citizenship evidence in separate mailings.

I would assume that the new passport card would have an expiration date that matches the passport book's expiration date.