In his retirement, he used this expertise to write a book on the subject, published in 1887 and entitled: A History of the Region of Pennsylvania North of the Ohio and West of the Allegheny River, of the Indian Purchases, and the Running of the Southern, Northern and Western Boundaries.

In 1863, he attracted statewide attention for a speech that he gave with the title Our National Constitution: Its Adaptation to a State of War or Insurrection.

His service on the Court included writing the opinion in the case of Commonwealth v. Drum, which is often cited as the foundation authority on the legal definition of murder in Pennsylvania.

[5] At the conclusion of his term in 1879, Agnew retired to his home in Beaver and from that point handled only a very few select legal matters.

[8] His father was a Princeton-educated doctor and his mother was part of the Howell family that was prominent in New Jersey affairs of that era.