There was a lot more going on here than a disagreement about claims in a book.
On one level, it showed conservatives' strange impulse to form a circular firing squad at the slightest provocation. Can you imagine Chris Matthews and Rachel Maddow getting into it like that on air?
The other thing is, Will views himself as a curator of All Things Reagan. Reagan made Will, who was a somewhat obscure syndicated columnist when Reagan took office. The first week in town, Reagan had dinner at Will's Chevy Chase Village colonial, thereby annointing Will as a Somebody and BSD with the Reagan WH, which had few media alliances.
I am sure Will was infuriated that a vulgar cable personality presumed to write a book, ok put his name on a ghost-written book, about the Great Man himself. Then the book apparently recycled old media gossip about Reagan's mental acuity. That is so not on. The fact that O'Reilly is kind of a fake conservative anyway yet acts like he is the last word added fuel to Will's rage.
For his part, O'Reilly has made a real effort with his books to establish some intellectual credibility. Plus, they probably made him a nice chunk of change. Now Will, the acknowledged Big Thinker among conservatives, disses Bill and his book. That had to sting and could leave a mark.
I thought O'Reilly got the best of the shouting match, but then again, I was astounded that Will even went on. Over the long haul, I suspect that Will prevails. O'Reilly has now been charged with a heinous crime in conservative circles, being disprespectul of the legacy of our Greatest President. Many conservatives don't like O'Reilly anyway, so he can't afford to piss off any angry old white guys. Not with Megan nipping at his heels.