"Fearless Fosdick," I'm sure you know, was a long-running strip-within-a-strip appearing as part of - and sometimes seemingly in place of - Li'l Abner. It's complicated. And that was part of its appeal.
According to the scholarly introduction by Max Allan Collins, Fosdick first turned up in a minor way in 1942 and soon wormed his way back to become an important part of Abner's world. This 1990 Kitchen Sink collection features five Fosdick stories that originally appeared between 1947 and 1952. A couple of them are framed within the Abner strip, as Abner reads and reacts to Fosdick's adventures, while the others, including one that ran daily for two full months, dispense with Abner entirely.
In the beginning, Al Capp was simply lampooning Dick Tracy, but the longer the almost-a-strip went on, the more Fosdick became his own man, and the more the satire was directed at society in general. What I find especially cool is that Fosdick inspired the great comic strip parodies Harvey Kurtzman and Will Elder did for MAD (the comic book incarnation) beginning in 1952.
Here's a small taste from the first tale in this book, "Introducting Anyface:"