Running With The Firm by James Bannon.
An undercover policeman infiltrates Millwall's infamous hooligan firm. The film ID was based on this book and there's some gripping stuff in there. How him and his mate nearly got caught being Old Bill, witnessing a Crystal Palace fan get beaten up in front of his family and not being able to do anything, friendships growing and the adrenaline of football violence.
I read a lot of football books but this one is up there with the very best.
Jamie still lives in Brighton I think. He's an interesting bloke. A couple of years ago he toured a one-man show based on his undercover experiences... went to New York I think. God knows what they made of it there!