Simon gave journalist Tom Bower ''dozens of hours'' of access during his research for 'Sweet Revenge: The Intimate Life Of Simon Cowell', despite his PR guru's advice to stay well away.
Max said: ''Tom only does what he wants to do, and the more controversial and sensational then the more it's going to sell.
''When Simon decided the best thing was to co-operate, he spent dozens of hours talking to Tom Bower.
''Of course I am disappointed. We've had a very close relationship for many many years, I think the world of the guy.
''But I think we, as a team, have scored a massive own goal by talking to him.''
Appearing on UK show 'This Morning', the publicist-to-the-stars said: ''It's open season. Yesterday six different girls called my office who supposedly have had relationships with Simon.
''I'm sure loads of girls will be contacting newspapers, many of whom have probably never met him or just had a picture taken with him. That will happen because of the Tom Bower book.''
Max admits he helped Simon and Dannii keep their romance quiet because it was what they both wanted, but believes Simon's decision to talk to Tom will hurt those mentioned in the book.
He said: ''He shouldn't have spoken to him in my opinion because it's given the book credibility from his own mouth and that will upset Dannii and everyone else.
''The fact of the matter is, in my view, Simon should have been in the position to say: 'Look, this has nothing to do with me, I've never met the man, I've never spoken to the man'.
''The big problem I have had with this is a lot of this is coming out from Simon's own words and this is a man who for 10 years I have protected and always been desperately keen to keep his private life private.''