The Hadano Library will accept donated books in English. They have a large collection of books in English at this very good public library.
Hadano Public Libraryhttp://opac.city.hadano.kanagawa.jp/images/top.bmp
… Hadano City Library WED,THU,FRI AM9:00 – PM7:00. TUE,SAT,SUN,Holiday AM9:00 – PM5:00 … 秦野市立図書館〒257- 0015 秦野市平沢94-1 TEL:0463-81-7012 FAX:0463-83-8370
Or you can donate them to your English school if you work at one.
Another idea is to sell them at Bookoff, which has branches all over Japan.