From the English Wikipedia page [1] - with more information on the German Wikipedia page [2]

The Wiesbaden State Library (German: Landesbibliothek Wiesbaden), formerly the Nassau State Library and Hessian State Library, is one of the state libraries of Germany. It is funded by the State of Hesse and located in Wiesbaden. With collections currently comprising over one million books, it is one of Germany's largest research libraries. The major special collection is on the historical state of Nassau, in which the library has its roots. As of 1 January 2011 it became part of the RheinMain University of Applied Sciences. The library building is situated in the Rheinstraße in central Wiesbaden.

The website is [3].

See also State libraries of Germany