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 library can be found South West of the Citadel. A Brotherhood Scribe inside will pay for any undamaged Pre-War Books the player can bring her.