Brand Luther: How an Unheralded Monk Turned His Small Town into a Center of Publishing, Made Himself the Most Famous Man in Europe–and Started the Protestant Reformation (very long title!) by Andrew Pettegree is a fascinating book which I recommend.
Though not written from a traditional biographical format, I learned quite a bit of new material on the reformer as well as gaining great insight on how the Reformation helped spread economical printing and how economical printing helped spread the Reformation. Both were essential for each other.
With the 500th anniversary of the Reformation on the horizon, I am sure that many new books about Luther and the Reformation will be forthcoming. If many of them are as superbly written as this one was, I can’t wait!