John , London-born bookseller noted for his volumes of Peerage & Baronetage (7)
I believe the answer is:
debrett
(Other definitions for debrett that I've seen before include "List of the British aristocracy" , "Founder of book on the peerage" , "Peerage reference book" , "Peerage guide" , "Man giving name to publishers best known for books on etiquette" .)