I think it would be a mesmerizing read and revealing in the same way On Writing was, which humanized him and gave us a biographical peek into his formerly shrouded life. There is a reason we enjoy hearing his voice in a Foreword or nonfiction work (like you mentioned) because it reminds us that he's just a guy who likes to read and write stories. I truly hope this happens one day. Maybe the "guys in the basement" will begin to move and he will once again be bitten by the nostalgia bug.
(I will add that King doesn't necessarily need to be humanized because he has the uncanny ability to close the distance with his folksy wit and charming candor, but I think his most ardent fans like to be reminded that he's just a regular guy.)