Mark Anderson showcases a 1966 cartoon book JEST IN PUN by Bil Keane. The cartoons are "Sidesplitters from Bil Keane’s Sideshow."
"Sideshow" was a reader-contributed topper to his FAMILY CIRCUS Sunday strip. The feature ran on the top of the FAMILY CIRCUS cartoon, hence the term "topper." Readers submitted puns to "Sideshow," and Bil would illustrate them.
From a 1975 Cartoonist PROfiles interview with Mr. Keane (via Michael Sporn):
"When my Sunday 'Family Circus' page started in 1′ appended it with a weekly panel 'Sideshow,' consisting of reader-contributed puns. The 25,000 letters a year that this feature drew, as well as the enthusiastic response other pun features, convinced me that thi [sic] many pun-lovers in the country. (And quite few in the city, too, but they won’t admit it.)"