Black Widow

She was first introduced as a Russian spy, an antagonist of the superhero Iron Man. She later defected to the United States, becoming an agent of S.H.I.E.L.D., and a member of the superhero team the Avengers

Recommended Reading
Black Widow: Web of Intrigue (Marvel FanFare # 10-13)

DareDevil V1 #187-190

Uncanny X-Men # 268

Captain America : Streets of Poison (Captian America # 372-378)

Black Widow : Itsy Bitsy Spider

The Widow (Daredevil V2 # 61-66)

Black Widow: HomeComing

Captian America V5 #27-50, V1 # 600-619

Black Widow: Kiss or Kill

Black Widow: The Name of the Rose

Secret Avengers # 16-21

Amazing Spider-Man: End of the Earth (Amazing Spider-Man # 682-687)

Winter Solider (Brubaker's Run)