An in depth analysis of the campaign, and the events surrounding it. Details the workings of both sides, and gives great detail (shell expenditures, planes lost to parts shortages), very good stuff if you're a war-nerd.
With the Old Breed: At Peleliu and Okinawa by Eugene Sledge
Gritty war memoir, one of the source materials for the HBO series The Pacific.
The Campaigns of Napoleon by David Chandler
Incredible depth, exhaustive, who knows who knows what, but after reading this you can give a cogent overview of the whats hows and whys of the Napoleonic wars.
The Thirty Years War: Europe's Tragedy by Peter H. Wilson
I've poked through a couple other books on the subject, none where as comprehensive or deep in their analysis. A little cynical maybe.
Hammer of God by Bo Giertz
Lutheran exploration of the Law/Gospel distinction, among other topics. Conservative pseudo-Christians beware, real spirituality here.