The 7 Lively Arts, (1924 is the first of many printings)
The movies come from America (1937)
The great audience (1950)
The Public Arts (1964).
He corresponded with T. S. Eliot, Clive Bell, Van Wyck Brooks, as part of his editing responsibilities at The Dial.
"The Krazy Kat That Walks by Himself," a chapter in The 7 Lively Arts, derives from the enthusiasm Seldes felt for this comic strip by George Herriman. The whole text of his essay on Herriman is here.
Michael Kammen has written The Lively Arts: Gilbert Seldes and the Transformation of Cultural Criticism in the United States (1996), to remedy the ignorance about "the first American intellectual to lend legitimacy to popular culture." I hope, if you hit this link, you will see one of the Christmas cards Herriman drew especially for the Seldes family to send out. (It is reproduced on page 97 of Kammen's book).