John Scalzi is a giant fan of H Beam Piper's classic science fiction novel Little Fuzzy (as am I — it's the first sf novel I bought for myself, from the used shelf at Bakka Books in Toronto, on recommendation from the writer Tanya Huff, who was working behind the counter then!). Such a huge fan that he's written a novel in the Fuzzy universe. He could do this because Little Fuzzy is in the public domain, so he doesn't need to clear copyrights. But having written the book — which is a loving tribute to Piper — he decided to have his agent show it to the Piper estate, who gave it their official blessing, making it part of the Fuzzy canon forever.
Color me impressed. And jealous. But mostly impressed. Go, John, go!
Little Fuzzy itself is in the public domain, but its sequels are still under copyright. While it might have been technically possible to write Fuzzy Nation without the permission of the Piper estate, because of the status of the sequels there were enough (forgive the pun) fuzzy legal areas that I didn't want to have to deal with them. Beyond this, because Fuzzy Nation is in many ways meant to be a tribute to Little Fuzzy and to Piper himself, I wanted the blessing, so to speak, of the Piper estate.
So, after I wrote Fuzzy Nation, my super-invaluable and incredibly awesome agent Ethan Ellenberg approached the rights holders to the Piper estate and started talking to them about it. The discussions took, well, a long time. But we reached agreement on it, and I'm happy to say Fuzzy Nation is an authorized work.