That's the title of today's post from Tyler Cowen both at his blog and as a guest blogger at Penguin. His point seems to be that the book you've read is likely not the best book you could be reading, and by passing it down the line (via donation or BookMooch or leaving it somewhere in public) your "gift" is preventing someone from reading something better. He says the calculations here are tricky; you could give the book to a friend, but if the friend is highly discriminating, then your standing in their eyes could suffer by proffering them a substandard book. Better to avoid those calculations and simply throw the book in the trash. The author has been paid, you've gotten what you want out of the book, and you've saved some poor schlub from having to make the calculations you made when you thought about buying the book in the first place.
His commenters are mainly book-lovers who beseech, implore, and adjure to donate the book to a library for its book sale, or a thrift store, or just leave it somewhere as a serendipitous gift for someone else. They also point out that Tyler may not know his friends as well as he thinks and that the second-bookstore or thrift shop would know better than he what value books have in their local market.
I go through periodic book purges. My usual method is to pile them up in a box (along with any CDs I've stopped listening to) and take them to BDFAR or Nice Price for trade. Whatever they don't take, I donate to the library for their book sale. And then the box goes back into the closet to collect more books, the making of which there is no ending.
I had a friend years ago who threw away an Anais Nin book because she thought it was so trashy she couldn't bear it anymore. I remember being astonished at the time (I was in my 20s) at the thought of throwing a book into the trash. Even for books I despised, I still would trade them for something better. Today, I'm still more likely than not to write in the margins and trade them if possible, even though I have less time today than ever to read books. My goal now is to either borrow them from the library or in some other way reduce the flow taking up room on my shelves, so that I reduce the time spent on purging them later.