History of Photography 

Oxford Art Online
- Art databases, searchable uniformly or individually, produced by Oxford University Press. 

Oxford Reference Online (includes photography)
~ Reference books migrated to electronic form.  Many art and photography sources not included above.

Encyclopedia of nineteenth-century photography / John Hannavy, ed.  2 vols. New York : Taylor & Francis Group, c2008.  Lilly LibraryReference R9 .H25 2008
~ newest reference source for the history of photography.  Scholarly, a good place to begin.

The Oxford companion to the photograph  Lenman, Robin. also hardcopy Lilly Library: TR9 .O94 2005
~ online and hardcopy starting point for photography history, terms, bibliography, etc.

The Abrams encyclopedia of photography / edited by Brigitte Govignon. New York : Harry N. Abrams, 2004.  Lilly Library Reference TR9 .P4813 2004 
~ smaller overview on photography, excellent plates.

Processes

The Focal encyclopedia of photography / edited by Leslie Stroebel and Richard Zakia  3rd ed.  Boston : Focal Press, c1993.  Lilly Library Reference R9 .F6 1993  
~ best handbook explaining processes, techniques and materials, historic and contemporary

Collections

International photography : George Eastman House index to photographers, collections, and exhibitions / Andrew H. Eskind, editor ; Greg Drake, Kirsti Ringger, Lynne Rumney, associate editors.  Edition  Enl.  3rd enl. ed.    New York : G.K. Hall ; London : Prentice Hall International, 1998.  Lilly Library  Reference  TR12 .E85 1998  
~ the most comprehensive indexes of collections and locations of individual phtographs and works.