Scribe Yearling

 is a member of the Capital Wasteland Brotherhood of Steel's Scribes. She can be found front desk area of the Arlington Library. She is related to the The Wasteland Survival Guide quest. Scribe Bigsley implies that she was assigned to the Arlington Library because she clashed with her superiors, most likely Scribe Jameson or Elder Lyons.

Unmarked quest: Yearning for Learning
When speaking with her upon entering the building for the first time, if she is spoken to nicely she will offer you 10 XP and 100 caps for each Pre-War Book you take to her.

Bugs
It has been known for her to just disappear and not be found again. She will move about the building to some degree, sometimes sitting at a counter upstairs and wandering around the corridors. She also has a bed on the second floor near the left staircase.