ABAA: Antiquarian Booksellers' Association of America, the national association of antiquarian dealers.
ILAB: the International League of Antiquarian Booksellers, an international organization of dealers.
SNEAB: Southern New England Antiquarian Booksellers, a regional association.
ABA: American Booksellers Association, the national association of new-book dealers.
LOC: The Library of Congress Web Pages, which continue to expand daily
There are a number of other sites of interest to the book researcher:
The American Library Association maintains a comprehensive Web site with information on their organization
Conservation OnLine accesses to resources relating to the preservation of books and manuscripts, located at Stanford University
Z39.50 Web Interfaces are a new approach to library research using Web forms. Try the Library of Congress Web Gateway.