Books?

So, we have a bunch of books in the attic to take back to the used bookstore.  The bookstore that we tend to go to has a (seemingly) decent selection of fantasy and science-fiction books.  However, I'm reluctant to get books at random because I feel compelled to finish books once I've started them...even if I don't like them.  So, what should we look for?

We've liked:  Terry Pratchett, George R. R. Martin (Ice & Fire, one book of short stories), Neil Gaiman, Margaret Ronald, Ursula LeGuin (the Earthsea books), Susan Cooper (I have a fondness for the Dark is Rising books; PoC hasn't gotten into them), Roger Zelazny (Night in the Lonesome October; The Changing Land, but we both hated Lord of Light) Orson Scott Card (the first couple of the Ender books--after that they got kind of dumb), Tim Powers (the Anubis Gate)....

I'd say we have a preference away from books that are all about battles in space, all about cool technology, or sooooooo far out there they're hard to fathom.

Suggestions?  Thanks!