One finds it challenging to get to all of the key pieces of the blibliophile collection of Aumale. Unique and invaluable manuscripts, early printed precious books, archives pieces, maps, richly decorated bindings, the Duke was attached, in his choice of purchases, to acquire only the most beautiful and precious in everything. When a more remarkable item than his own went for sale, he would not hesitate to reform the old one and to add the new one to its collection. Even during his successive exiles, Aumale stayed in touch with the auction room world through various buyers who sent him the catalogs, which he returned to them with personal annotations. The Library preserves these catalogs, which gives better insight into the acquisition policy of the Duke. Leafing through the catalog, mentions such as "To buy whatever the price" can even be discovered, as well as every single sale price. Aumale was therefore able to stay completely atop the nineteenth century book market, arguably a great period for a bibliophile such as Aumale, who had the culture and the means to build the most beautiful collection and to leave us with unprecedented information.