69 A.D. : The Year of Four Emperors by Gwyn Morgan

Fairly dense history of the civil war that the title implies. Morgan is attempting to take the few but disparate sources and work out what was most likely to have actually happened. The paucity of material means that a lot of this is fairly speculative (or relies on just one original author). On top of this, the number of people and legions involved means that just keeping track of those is hard work in itself.