Mail your books to Books for Prisoners. Prisons and detention centers are always in need of used trade paperbacks. They accept all types of books except those of a sexually explicit nature. Boxes of books must not exceed fifteen pounds. If you have books that are in different languages, those are needed as well-- especially Spanish and Russian trade paperbacks. Books must not be stained in any way or they'll be rejected.
Send your books to a soldier overseas through Operation Paperback. Trade paperbacks are easier for a soldier to pack in his knapsack than a hardcover so they're preferred. Operation Paperback is a nonprofit organization that has sent over 1.1 million paperbacks to soldiers away from home. Donations of any quantity are accepted.
Drive to the hospital with your used trade paperback books. Many hospitals appreciate having used trade paperback books available to those in waiting rooms who have forgotten to bring reading material. Call the hospital first to make sure book donations are allowed.
Call your local homeless shelter or women's share house and ask if they're accepting donations. Young children often stay at these facilities with a parent; therefore, trade paperbacks and used children's books are a valued commodity.