Anybody ever do/has done this?

We get sent a lot of stuff at work that is meant for the people that used to be here. When enquiring about what I should do with said post, I was told "open it. If it's good, keep it. If it's rubbish, throw it away"

Which seems a bit out of order, but seeing as we've no idea where they've gone it also seems... acceptable

So I've just opened a package. It's a crappy second hand book, written by Derek Rowntree. 'Statistics Without Tears - An Introduction for Non-Mathematicians'

Problem is, another look at the address revealed it's for the people next door, not the people that used to be here. I didn't realise until the book was in my clammy opportunistic hands. How am I supposed to give it back without looking like a thief?

"Hi, I was rooting through your post earlier, but as it's not worth anything I might as well give it back"

Ever got anything GOOD through the post that wasn't meant for you?