There are few that have pretty, elegant covers, but have been absolutely terrible. CoughSense&Sensibility&SeaMonstersCough.
But for the most part, I like finding old editions of books I love. Even if I already own a copy. It's nice to have a spare to lend to someone.
I always feel like Mel Gibson in "Conspiracy Theory" when I can't pass a copy of F.Scott Fitzgerald's "This Side of Paradise" or any Sherlock Holmes Collection. Each copy has its own personality for me.
The set shelves in my room is quickly overflowing. That always tends to happen after our local library book sale. I need to de-clutter it a bit so the bowing shelves do not break under pressure. Oh, but there are so many things from around the world that friends have given me or I have been fortunate enough to acquire in person that only hold sentimental value to me and would probably be worthless to anyone else. It's difficult for me to see that shelf and not feel bombarded with good memories.
What treasures do you have on your shelves?