As the 25th anniversary of John F. Kennedy Jr. and Carolyn Bessette's death approaches next month, DailyMail.com columnist Maureen Callahan's new book has shed fresh light on their troubled relationship.

In 'Ask Not: The Kennedys and the Women They Destroyed', Callahan tells how, on the day the couple died – July 16, 1999 – their marriage was effectively over. 

'John had moved out of their marital flat,' Callahan writes. '[But] the last thing he wanted was the press speculating about his marriage.'

And so, JFK Jr. insisted on piloting Carolyn and her sister Lauren to a family wedding. Off the coast of Martha's Vineyard, Massachusetts, he crashed his small private plane, killing them all.