Libraries, bookshops and museums:

ZVAB is an interesting website for finding antique books & prints. http://www.zvab.com 

NCC is the combined online catalogue for the Dutch national and university libraries. Useful to get an impression of how rare a book is and its collation (signature). http://access.authkb.kb.nl  You can become a member for a small fee.

The prime library in the Netherlands is the Koninklijke Bibliotheek (royal library); http://www.kb.nl

The catalogue of the British Library is:  http://catalogue.bl.uk (includes an interesting database with photo's of bookbindings.)

A key source for books and maps in the United States is the Library of Congress) http://catalog.loc.gov/ 

Website of the book musum Meeermanno Westrenianum in The Hague  http://www.meermanno.nl/