Can one cancel a book that has been marked mailed? I was not at home when I marked it mailed, was sure I had the book, but someone swiped it from my shelf and I don't have it. PM'ed the person immediately, but no reply. Can I cancel it?

I believe the only way to cancel is to use the Contact Us link at the bottom of your screen and send a Feedback email to the PBS team so they can cancel it.  Lesson learned: Don't mark books mailed until they are.  :)

Once it's been makred mailed only PBS can cancel the transaction.  You can let it go lost and the requestor will get their credit back but it's not really fair or nice to make them wait a month.  I would contact PBS so they can cancel it earlier. This is why I never  mark a book mailed unless I've varified that I have the book and it's still postable.  Usually I won't even print the label.  But I have easy computer access everyday.