All my books are hardback, no one collects paper backs, ..well saying that if its a proof or ARC they are normally paper backs, but rare, I have managed to get a spare proof for the latest gemmell book and there are only 50 of them anywhere.
Most of the books i have collected over the years, although now i know a few authors and i sell for them, or i buy in from them and get them signed eg simon scarrow i got him to do a hand numbered limited edition of 30 books, and we applied a special stamp inside the book to make them stand out from the norm, they sold like hot cakes.