More used books
Bill in Yokohama is wondering if there is any individual or organization in Japan to which he might donate used English titles. "The books are not textbooks, but rather works of fiction and nonfiction in good condition."
You could try your local library. If there is a section already dedicated to English language books -- hardbacks and paperbacks -- there is a good chance staff will be happy to take them off your hands. If there is no section, maybe there should/could be? If such a system is operational, often there is a day or time in a day set aside for receiving donations.
Alternatively, there are now two bookstores dependent on taking used English-language titles aboard.