Despite its title, this multivolume reference work is really an encyclopedia. Monumental in scope, it is the largest and most comprehensive reference work on the visual arts that has ever been published. Don't start any research on art without checking this source. It is also available online in Oxford Art Online.
Although older than the encyclopedia above, this is still a useful and scholarly work. It is richly illustrated with thousands of color and black-and-white plates, which are all found together at the back of each volume.
The first of these encyclopedias is a multivolume work containing scholarly articles on all aspects of architecture. The second is a beautifully illustrated one-volume encyclopedia devoted to American architecture of all time periods, including the nineteenth century.
These are excellent places to begin research on photography or photographers. The Encyclopédie, which is written in both French and English, provides useful chronologies and bibliographies and is a good place to start research on a particular photographer.