File this one under "you learn something new every day".
Bob Dunn is reasonably well known in the world of comic strips as the cartoonist who took over Jummy Hatlo's They'll Do It Every Time after Hatlo's death in 1963. Dunn won several awards from the National Cartoonists Society at the end of the 1960s. Long before that, however, he produced this book for Western Publishing in 1936. It is a collection of more than fifty single-page gag strips - each one a knock knock joke. They are all in black-and-white, and most are five or six panels since, well, knock knock jokes have five parts (the sixth panel is generally one in which the jokester gets a punch in the nose for telling such a poor joke). The work in here is pretty accurately reflected by the card stock cover - they are minimal, sketchy drawings. Not a lot of detail, not a lot of craft, not a lot of wit. Lots and lots of racist imagery, which (sadly) almost goes without saying for the period.
When I looked up Dunn, I found that Maurice Horn's The World Encyclopedia of Cartoons credits him as the inventor of the knock knock joke. That surprised me. Partly because you never really anticipate that anything as simple as a knock knock joke has an "inventor" (though, of course, it would have to), and partly because I would've thought that the knock knock joke would have been around longer than eighty-two years.
Looking up the knock knock joke, seems to give some credence to the claim. Wikipedia traces the form back to Macbeth, but, well, please. They cite a couple of examples from before 1936 that are akin to knock knock jokes, but which lack the precise five-part form. Then, all of a sudden, they have a surprisingly large number of examples from... 1936. The most interesting of these, to me, is the fact that Fred Allen complained about the "negative craze" in his 1936 year-in-review broadcast. So it evidently was a new idea.
More interestingly, the knock knock joke seems to have faded away and then been revived by Rowan & Martin's Laugh-In, suggesting that the form faded away, and then was revived by a terribly smart tv show with an ironic and retro tone, and that it then stuck in our culture. As the father of a still-young son, I can assure you that knock knock jokes still have currency and still haven't improved very much.
Horn called Knock Knock "a million selling book". I have no idea if that's true. It wasn't terribly hard to locate nor was it expensive for a comic book from the 1930s, so it may well have been ubiquitous.