Aachen

University Library RWTH Aachen (free of charge)                                                          

Cologne 

University and City Library of Cologne (20€/year, refundable)              

TH Cologne Library (free of charge)                                                                                

Düsseldorf 

University and State Library Düsseldorf (15€/year)                                                      

Mainz 

University Mainz Library (15€/year, refundable)