He gave Félix-Hilaire Buhot his first one-man show in 1888,[2] and about the same time started to buy and sell a large number of Whistler's prints.

There he sold etchings and engravings by the Old Masters and works by Modern artists, such as Jean-François Millet, Félix Buhot, James McNeil Whistler, Edith Loring Getchell and Joseph Pennell.

Keppel sold prints to major New York collectors such as Samuel P. Avery and Mary Jane Morgan.

In the following year he translated Alfred Lebrun's catalogue of the etchings, heliographs, lithographs, and woodcuts done by Jean François Millet, New York 1887.

[3] In 1910 many of his articles and lectures were compiled into his book: The Golden Age of Engraving: A Specialists Story about Fine Prints.

[5] He wrote articles and pamphlets to accompany exhibitions at his gallery, and often gave lectures on subjects related to prints.