I will recommend this book with some reservations. "Handcuffed to a Corpse: German Intervention in the Balkans and on the Galician Front, 1914-1917." by Michael Kihntopf. It is a good English language treatment of the German general August von Mackensen and the conquest of Serbia and Romania. It also looks at how hapless the Austro-Hungarian army was in World War One.
This book does not have good reviews on Amazon.com So I do not recommend that you buy it. $25 for a thin, 113 page book is not a good value for the money. Kihntopf does not have any German language sources in his bibliography, which is a serious short coming for any book on the Eastern front of World War I. There are also some minor problems with spelling the names of the actors and places that cause serious students of World War One to grind their teeth. There are better, more recent books that look at how poor the Austrian-Hungarian Army was in World War One. While I have not read them, the Wikipedia article on August von Mackensen lists three German language books that are far more substantial. I do not know if they are any better, but they definitely have more details on this obscure but still very important German general.
D J Goodspeed's biography on Ludendorff is an excellent source of information on Max Hoffmann, von Hutier and von Francois.