I've just finished this interesting book by John King. It is an insight into the problem of football "hooliganism". The main character Tom Johnson is a Chelsea supporter and a member of what he calls a Chelsea "firm", a group of hard men out for the adrenaline of a punch-up with a rival "firm" from an opposing team. The most violent chapters are Tottenham away and Millwall away (in the latter Johnson ends up in hospital). It's an interesting commentary on the rivalry between the various London clubs - not much I know about since where I come from it is City v United.
Some people blame alcohol for the problem, but significantly, the firm only has two pints before their Millwall fixture - enough to give them an edge, but not reduce their fighting ability.
It's also untrue that they're not interested in the football itself - they do go to the match and there is very little trouble described inside the ground. The scraps are all on the streets.
I believe it's also been made into a film.