Tony,
That is an easy one: 'The Austro-Prussian War' and 'The Franco-Prussian War' both by Geoffrey Wawro and published by Cambridge University Press.
Both great books, Wawro has a writing style that shifts smoothly between the strategic and the tactical and moves at a pace that leaves you feeling like you are part of the action. Amazons stocks both of these books. Highly recommended.
Mike