Ask the staff at a local nursing home or retirement facility whether their residents would appreciate a donation of your old encyclopedias. Workers who know the interests of their residents are in a position to tell you whether your donated encyclopedias would be an appropriate addition to the facility's break room or library.
Put your old encyclopedias in a box and drop them off at a local charity's thrift store. Workers process the donated books and then put them on the store's shelves, where customers searching for a good deal, sometimes better than what they can find in conventional book stores, can take advantage of your generosity.
Contact your local school. Ask someone in the administration building if your encyclopedias would be a welcome addition to a classroom, or if the library has a need for them. Deliver the encyclopedias to the school if your offer is accepted.
Visit your local public library and ask a librarian what the procedure is for donating used encyclopedias. Your encyclopedias might find a place on the library's shelves or in the bookstore of a "Friends of the Library"-type organization.
Display your old encyclopedias on a table in front of your home while you hold a garage sale. One of your neighbors may find the used books to be an attractive deal and scoop them up.
Call local bookstores that sell used books. Ask to speak with the store's used-book purchaser. Describe the old encyclopedias, including the name, publisher and date of publication, as well the books's condition. If the purchaser is interested in the encyclopedias, she may invite you to bring them to the store for evaluation and possible purchase. When a buyer shows interest, ask if you can get more in store credit than in cash for the encyclopedias; you can then use the credit on other books in the store.