I've just had someone send me a PM saying that the post office wrote "unknown address" (or something similar) on the book they sent to me, which was obviously returned to them.

The address in the system is correct. I just received 3 books yesterday, so I know that the post office gets it right most of the time. It's not a PO box, it's not an apartment, and it's not in a small town that might be hard to find. Literally, the address is 15 feet from the actual post office!

What should I do? Should I call up the local post office and ask them what the problem is?

Update: Problem has been solved. The person who sent the book called the post office and they offered to resend the book for her. I also talked to the local post office and they said that it was just a fluke and reassured me that it would never happen again.

I would contact the local post office and tell them what happened. My guess is it was delivered to the wrong address and that person wrote on it. Some people write all sorts of stuff on mail that does not have their name or address or both. It can be then misinterpreted by the postal employee that sees it and returned. Hard to tell without seeing the packaging.

This happened to me. I sent a "care" package to my son who is in college but it was returned as "attempted not known". I don't know how this happened as my son has never had a problem receiving his mail. I took the box back to the PO and they resent it free of charge (I was happay about that because it cost $30 to send the first time). I guess it just happens sometimes (they could give me a reason why it was returned either).