...But I've already received it and read it! I sent the person a PM to please mark it sent so I can mark it received, and the mail-by date was yesterday. So, I assume with the two-day grace she has until tomorrow to mark it sent?

If she doesn't, where does that leave this transaction?

You may be able to mark it received from your Transaction Archive if ti's not on your account page anymore.  That way the sender gets credit.  Make sure you cancel any other requests for that same book so you don't get 2 copies. 

Did you confirm that the person on your request page that hasn't marked it mailed is the one who actually sent it? If someone else mailed it and failed to mark it mailed, the request would have been rolled over to the next person with the book. If the name on the packaging does not match the one on your request tab, then check your Transaction Archive for a cancelled transaction for the book and mark it received from there. If the name on the package does show it is from the person in your request tab and they don't answer you, you will have to wait until the system times out their order and mark it received from the TA then. Just be watching for the system to request it again for you and cancel any new orders for it.

Thanks :)

Yup, double checked all the user info against the address on the package before I sent the PM... Now I'm just waiting for her to respond or the request to time out (looks like it should time out tomorrow sometime) and it's a WL book, so I'll just yank it back off my WL once it times out.