Credo Reference is a collection of many reference books grouped together for cross-searching. Includes:

Aesthetics A-Z
A Biographical Dictionary of Artists, Andromeda
The Bloomsbury Guide to Art
The Bridgeman Art Library Archive
Critical Terms for Art History
The Encyclopedia of American Folk Art
If the Paintings Could Talk National Gallery Collection
The National Gallery Companion Guide
The Thames & Hudson Dictionary of 20th Century Architecture
The Thames & Hudson Dictionary of Art Terms
The Thames & Hudson Dictionary of Art and Artists
The Thames & Hudson Dictionary of British Art
The Thames & Hudson Dictionary of Design Since 1900
The Thames & Hudson Encyclopaedia of Impressionism

Oxford Reference also contains many reference books that can be searched simultaneously:

The Concise Oxford Dictionary of Art Terms
A Dictionary of Architecture and Landscape Architecture
A Dictionary of Modern and Contemporary Art
A Dictionary of Modern Design
The Oxford Companion to Architecture
The Oxford Companion to the Photograph
The Oxford Companion to Western Art
The Oxford Dictionary of American Art and Artists
The Oxford Dictionary of Art
The Oxford Dictionary of Art and Artists
 

Who's Who and Who was Who is a directory of over 100,000 entries for the "noteworthy and influential in all walks of life, in the UK and worldwide."