The Hector Hodler Library in Rotterdam, which is devoted to this artificial language, has over 15,000 books
[DOC] [TLE] Artificial Languages, Universal Languages | The Sheridan Libraries ... [PAR] Jan 28, 2016 ... And most of us have heard of Esperanto (a "universal" language), and other ... 
and WorldCat show quite a few more books about artificial and imaginary 
languages.