Architecture 

Oxford Reference Online - Art and Architecture
- variety of Oxford University Press online reference titles, many which include architecture (usually artistic and historic aspects).

Oxford Art Online
- Oxford's combined electronic art databases, including the (37-volume) Grove Dictionary of Art and the Encyclopedia of Esthetics

Encyclopedia of World Art - hardcopy only
- older, but half of every volume is plates (pictures), may in color and not on the internet

Pelican History of Art (series) Lilly Reference [various titles depending on period] Lilly N5300.P38
- the English-language history of art, including architecture.  Great Images. Great for contextualization.

Encyclopedia of 20th century architecture. 3 vols 
Lilly Library Reference NA680 .E495 2004
- short but scholarly discussions of buildings (Hysolar Research Institute), bios of architects (Rem Koolhaas), concepts ("Utopian Architecture")

The Greenwood encyclopedia of homes through world history. 2009  6 vols.
Lilly Library stacks and Reference NA7105 .S74 2009
- terms, buildings, countries or regions and their architecture

The Greenwood encyclopedia of homes through American history. 2008 4 vols.
Lilly Library Reference NA7205 .G745 2008
- American focus on all aspects fo domestic architecture

Phaidon Atlas of Contemporary World Architecture. 
Lilly Referance Atlas Case
- spectacular huge 'atlas-style' book on modern and post-modern architecture.  Includes anxiomatics, color pics, written history, etc.  Great fro scanning.  Basic and thorough.