The British Library's Philatelic Collections(site info)
A collection of stamps, postal history and books, including the Tapling Collection. Over eight million items. The collections, together with philatelic books, photographs, general reference material, maps, newspapers and official publications elsewhere in the Library, make the British Library one of the world's prime philatelic research centres.
http://www.bl.uk/collections/stamps.html
Rocky Mountain Philatelic Library(site info)
Houses over 5000 books on philately, issues of periodicals, auction catalogs for research purposes, a western history (postal history) and railroad collection, post office publications dating back to 1876, a collection of maps dating back to the 1870's, and an education program for children and beginning adult collectors. Denver, Colorado.
http://www.collectors-mall.com/philib/RMPL.htm
American Philatelic Society(site info)
The largest public philatelic library in the United States, Its holdings include catalogues, government documents, auction catalogues, and a variety of other materials. Located in State College, Pennsylvania.
http://www.stamplibrary.org/