How do you decide when to throw away a book that you haven't read if you are running out of space?

[Carol]

I have permanent books and "read and pass on" books. Permanent include reference books, autographed or books by a favorite author, a series I particularly like (like Outlander & Grafton's Alphabet series). There are others I read and then pass on, either to Rose's Books (our used book dealer where I can trade them in for different books or get credit for future purchases) or to friends. Also beat up books go or ones I never read and have been sitting on the shelf forever.
CJ

[Cheemoandia]

Or post them on websites such as bookmooch, paperbackswap, or swaptree. Other would enjoy them, and who knows you might find something interesting.