See Books about Cornell for campus building information and photos on campus (especially Parson's 1968 The Cornell Campus). For a concise history of a particular building, try searching Cornell Fact Search (aka "Super InfoBase").
For students conducting research on specific Cornell buildings, information on locating the building plan numbers is available at Rare and Manuscripts reference.
Building plans are available are also available to students working on projects from the Facilities Information Group--part of Facility Services, Humphreys Service Building. See http://fe.fs.cornell.edu/fig/archive.cfm for the policies and procedures governing access to these plans. See Procedures for Student Projects including a link to the approved buildings list.
The University Archives in the Rare and Manuscript Collections has a clippings file on a large number of specific Cornell buildings; ask at the RMC reference desk. Also in RMC, the Cornell Clippings File and the ClipFile Index has information from local newspaper clippings about Cornell buildings (dedications, fires, demonstrations, etc.). The Olin reference vertical file (at the reference desk) has a small folder for Buildings.
For exhibits and tours of library buildings, see the Digital Collections section of this guide.