I have a book coming to me that will go lost on Sunday.  I know that a new book will be ordered for me (it's WL) and the sender is out the credit, as they didn't use PBS's DC. 

My question is, do lost books count as marks against you?  I'd hate to get marks against me because a book never arrived. This will be the first one I've ordered that went lost...

 Sorry to hear that your book got lost,.Things happen, unless you have a lot of books go lost you won't get a bad mark for it. I wouldn't let it worry you since it's your first one.

The only time eyebrows are raised is if you repeatedly have books "go lost."  Then there becomes suspicion that you are just not marking them received.  But I wouldn't worry about one or two or even three.  Also, I have had books "go lost" only to arrive past the deadline...one was a month late!  If it does arrive, you can mark it so in your transaction archive.