Once the idea of following the career of the expelled bully from Tom Brown's Schooldays was conceived, what made them work so well is that most of the books was true, genuine history no matter how incredible it sounds now. GMF would move a quote around here or there (a quote from one general was made in a different battle, for example), make a judgement about which account of a particular episode was probably most accurate (like who was to blame for the Charge of the Light Brigade), and stick Flashman in it (in more than one sense!)
If you haven't read them, you really should.