The Hector Hodler Library in Rotterdam, which is devoted to this artificial language, has over 15,000 books
The Hodler collection contains around 20,000 books... [DOC] [TLE] Hector Hodler Library - Wikipedia [PAR] The Hector Hodler Library is one of the largest Esperanto libraries, with 
approximately 30,000 books, with periodicals, manuscripts, photos, music, and 
other collections.