I sent out two books wrapped together. I noticed today that one of the books was marked received, but not the other one. I am going to give it a few days to see if they mark it later, but what can be done if they do not?

I would PM them and politely remind them that you sent two books together and that one has not been marked received.  Make sure to ask them for a response within a timeframe (I'd give them 5 days or so), then if you do not hear from them by that time, contact the site owners and explain what happened.

I think I remember reading on here the other day that someone received two books together but, only one was on their list to accept, they were trying to figure out what to do...hmmm maybe that is your person...

Also, sometimes people forget that they have to mark BOTH books, thinking of it as one transaction.  I go ahead and send a friendly PM now.

I would send a polite PM.  I was guilty of this once. Someone posted 2 WL books to me.  Because they were WL it put them on seperate transactions but she wrapped them together to save on postage.  I marked one and it didn't even occur to me to mark the other one received.  She PM'd and reminded me and I had a d'oh moment.  It's very likely that the recipient thought because they came together they only had to mark one as received and haven't noticed the other book sitting in the enroute to me tab.

Ok....thank you everyone for your help :)