There are lots of resources dealing with the Allied perspective and the various famous battles, but there is precious little available in the form of competently researched and written general overviews of the Italian Campaign from the German's perspective (unlike Normandy or the East). One almost has to piece together Axis intentions and dispositions at various stages of the campaign based on unit histories and the scraps Allied-centric authors devote to them.
Kesselring's own memoirs are very informative. He was not the best writer, and the rather poor English translation of his work compounds its sometimes ponderous nature, but his analyses of German and Allied strategies in Italy are excellent and really contrast the rather rosy description Alexander posited through the London Gazette in 1950 and the absolute fluff found in Clark's Calculated Risk.
Monte Cassino: A German View, by Rudolf Bohmler, obviously isn't focused on a grand overview, but it does detail the campaign from the German's view before and after the battle much better than your typical Osprey type single-battle-focused book. (We're talking chapters, not pages.) It's a bit rare these days, but not impossible to find.
Also, here is a very broad overview of the German strategy in Italy and how it played into the war effort as a whole.
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