A Strange Kind of Glory by Eamon Dunphy is about Matt Busby but also looks at the slightly shady nature of pro football from its earliest days.
Colin Shindler's study of what happened to the youth players who played in a Man City v Utd match is excellent. George Best and 21 Others is the title I think. Covers the uncertainty of making the grade very well.
Dunphy's diary account of a season as a Millwall player Only A Game? is also very good.
Morbo , a history of Spanish football, by Phil Ball is a good read.