if you search by the ISBN it's available on Amazon, but it's unknown title, unknown author: https://www.amazon.com/Untitled-Nonfiction-SM-Be-Confirmed/dp/1501194291.
You're right that he's alway saying stuff like that, but I do remember recently him saying to someone that he's going to their show because he has a project to promote. I forget who it was.
Edit: it was the Gwyneth Paltrow interview. He said "he may have something to promote soon"
I didn't say it was good, just saying it wasn't universally panned. Look at the Amazon reviews. Some people liked it.
Stern released a book of his interviews recently where he definitely downplayed his shock jock persona in favor of him being a "serious" interviewer.
Howard Stern has major F-U money these days, and I expect him to retire soon - I think he's trying to rehab his legacy before then.
It's true. Coming May 14th. Here's link to Amazon...
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The last update I provided indicated the book is 544 pages and will be released in hardcover and Kindle on March 19, 2019.
https://www.amazon.com/Untitled-Nonfiction-SM-Be-Confirmed/dp/1501194291