Arlington Library

 a former forward operating base of the Capital Wasteland Brotherhood of Steel, transformed into the epicenter of a project to recover and store pre-war books.

Though confusing to find, it is easily accessible from the Jefferson Memorial, once the memorial is located as part of the main quest.

Fast travel to the memorial and swim west across the river, towards the boat. There is a dock landing to climb out on. From there it is a straight shot to the library.

Apperances
 only appears in Fallout 3.

The liabary can be found South West of the citadel (BOS) headquarters for the Capital Wasteland. There is ways of also making good money here finding books for the BrotherHood.