John Carter's ABC for Book Collectors: A free download (as a .pdf) of the classic reference work on book collecting and terminology.
The Antiquarian Booksellers Association: The oldest bookseller's association in the world, founded 1906, the ABA is dedicated to maintaining high standards of both cataloguing and customer service.
The International League of Antiquarian Booksellers: ILAB has 23 member National Associations including the ABA, representing the world's finest booksellers. Their website provides many resources for bibliophiles including a searchable database of some 2,000 leading booksellers.
The Provincial Book Fairs Association: The PBFA organises numerous regional bookfairs every year.
ESTC or the English Short Title Catalogue: Providing an extensive catalogue of books published before 1800, searching over 2,000 libraries.
COPAC: The online union catalogue of major British libraries.