Yeah, it seems to me the main reason these books were published is because he was already well-known for his newspaper columns. If you're a "personality" in a medium outside of books, the chances of getting a book published go up greatly.
Syndicated talkshow host Neal Boortz wanted to write a book on, of all things, taxes, and tells the story of how he and the publisher agreed that, since they didn't really expect a book on taxes to sell that well, that he'd sign a two-book deal. "The Fair Tax Book" co-written with US Congressman John Linder turned out to be a best-seller. Boortz' second book of the deal was "Somebody's Gotta Say It" appears to have done well, though perhaps not as well as the Fair Tax book. That book is a collection of mostly-unrelated chapters ranging from his persona and political views to reminiscences of his earlier days in talk radio (the "cat catching contest" episode is hilarious), and doing other activities such as flying a hot-air balloon.
It seems most every radio talkshow host you've heard of has published at least one book. And there are many others who had their "fifteen minutes of fame" and got a book publishing deal out of it. Lisa Beamer's "Let's Roll!" comes immediately to mind.
Sure, there are unknowns who get published, and some of those books become popular, but having a large audience in another medium obviously greatly increases one's chances. If you're going to write "a book" like that, you may want to write and submit "sections" of it to newspapers to see if you can get any published, and especially see if you can get a regular column.
Another "humorist" perhaps along the likes of Grizzard is Ludlow Porch, who did talk radio decades ago (as mostly non-political, I vaguely recall listening to him back them), may have also been a newspaper columnist, and who published several books, but he was perhaps popular only around Atlanta, whereas Grizzard achieved a wider audience. One Porch title I recall is "Can I just do it until I need glasses?"
