The Gaoth Dobhair clubman left after a meeting with the team management including Jim McGuinness.
It is believed Donegal were unhappy with comments made by Cassidy about the team in the book.
Cassidy had outlined how he successfully quelled a potential players revolt in 2010, the year before Jim McGuinness took charge.
Cassidy, who was captain that year, revealed the detail in a book called ‘This Is Our Year’, which traces the lives of players from all nine Ulster counties this year.
Painting the background to his decision to retire after the 2010 season, a decision he subsequently reversed after a two-hour conversation with McGuinness, Cassidy said players had lost respect for the previous management team headed John Joe Doherty.
The extract from the book has drawn a firm response from one of Doherty’s selectors, Tony Boyle, who has contested Cassidy’s opinions of their set-up.
Cassidy suggests in the book, which was written by Declan Bogue and that he saw little point in any uprising as their season ran out of steam.
“A lot of players had lost respect for John Joe by that stage. Not as a person, just their belief in his team. As captain, I was trying to hold things together. A lot of my friends on the panel were saying to me, ‘f*** this’.
“My argument would have been, ‘Look, we’ve put in so much effort, there’s no point throwing the dummy out of the pram now. Let’s play it out and see how it goes’.”
Cassidy was a county hero just a three months ago when he scored the winning point in the dramatic All-Ireland quarter-final win over Kildare.
Tonight Jim McGuinness confirmed that Cassidy had left the panel but would make no further comment.
The Gaoth Dobhair man had defended his participation in the book, asking people to judge him when they had read all of it.
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