I was triggered to do some research after an offhanded comment while watching the West Wing.
Some basic googling has shown that the then-small Library of Congress was lost with its 4000 volumes and that White House staff were able to save some key documents like the Declaration of Independence and letters from GW.
While it likely was no Lighthouse of Alexandria, did the British looters and raging fires destroy or disappear any important historical artifacts?
There's a famous story of First Lady Dolley Madison saving White House paintings from the fire, but one of the paintings lost in the fire was a portrait of the Miami Nation chief Little Turtle by the artist Gilbert Stuart. This comes up in the Congressional Record during a floor speech by Indiana Congressional representative Souder, who is talking about the efforts for the tribe's federal recognition [source: 146 Cong Rec H8897 on Oct 5, 2000 ; see http://www.gpo.gov/fdsys/pkg/CREC-2000-10-05/html/CREC-2000-10-05-pt1-PgH8896-2.htm ]
There is also a reference to some missing Census records from the 1790 Census (First Census) as a result of the burning of the Capitol building: "Returns for Delaware, Georgia, Kentucky, New Jersey and Tennessee were destroyed when the Capitol building was burned in 1812." [see U. of Texas Libraries: http://www.lib.utexas.edu/government/census/census_1790.html ; also see NC State University Libraries: http://www.lib.ncsu.edu/guides/popcensus/1790 ]