This is Elihu Vedder's autobiography The Digressions of V - Written for His Own Fun and That of His Friends published in 1910, the year Winslow Homer and John La Farge died. Vedder and Homer were good friends starting when Winslow Homer first went to New York.
In the quote below, Vedder is writing about the time period during the Civil War when he needed to supplement his income with various extra jobs. When Vedder refers to "the Boys," he is referring to his friends during that period, which would have included Winslow Homer. Vedder wrote,
Then came the period of comic valentines. These were horrible things, but drawn on graphotype blocks were cheap enough to suit the publisher; but the funny thing about it was that he insisted on my making the verses -- poetry, he called it -- as well. He said: "You artists can make anything but money." Here I called on the Boys, and we set to work writing them and had great fun, for we instilled our stories and personal jokes into these things, which all passed undetected by the good publisher, who thought the "poems" fine. (from page 198)Winslow Homer probably wrote some of those cards with Vedder.