The artists knew each other and their unusually realistic still life paintings were derisively dismissed even during their own lifetimes. Harnett was one of the more successful of the artists painting in the trompe l'oeil genre, but even his paintings were given the snub by critics as mere novelty and trickery.
Trompe l'oeil is a genre of still life that exploits the peculiar nature of human perception to create the specific illusion of reality of the painting's subject or objects. Unique to this genre of realism, it follows very specific principles to achieve the illusion of reality. For example, Peto's and Harnett both painted the objects in their works as their actual size. And the items in the paintings rarely were cut off by the edge of the painting, as this would allow a visual cue to the viewer that the depiction was not real. But the main technical device was to use the shallow depth cast by the shadow of the objects to suggest depth without the eye seeing actual depth. Thus the term trompe l'oeil - "fool the eye." Books that discuss Harnett's and Peto's paintings rarely explain the technical achievement in these paintings, but the insight of the artists into how to achieve the extraordinary reality of objects depicted must have been profound.
The subject matter of the paintings was invariably of the most ordinary kinds of things -- pistols, horseshoes, bits of paper, keys, books, etc. Peto specialized in old time "letter racks," which was a kind of board that used ribbons tacked into a square that held notes, letters, pencils, and photographs. Harnett worked a little larger, painting pistols, horseshoes, hats, walking sticks and horns. Harnett's famous series of paintings depicting items from "After the Hunt" show muskets, dead game, hunting equipment, etc. are beautiful and exuberant in their detail and reality. The "After the Hunt" paintings were rightly praised as great art but nevertheless became lost to history for a time.
Peto's paintings are generally considered less technically skilled as Harnett's but their other aspects more than make up for the technical deficiencies. They are more abstract, use more unusual color, and often have a stronger emotional resonance. Both artists deserve more attention as painters that could instantly enthrall the viewer with a disturbing but pleasant sense of confusion.
The best resource to learn more about Peto and Harnett is Alfred Frankenstein's After the Hunt, William Harnett and Other American Still Life Painters 1870-1900. Frankenstein's book itself is a fantastic tale of solving the mystery of why these artists were forgotten for over a hundred years.