Starting from a point where I only knew about Stauffenberg what school taught me, this is the most interesting of the lot I picked up at the library, because it doesn't describe the event and personae superficially but digs deep into the personal and family history of Claus and Berthold von Stauffenberg. Looking at Claus Stauffenberg from the year 2007, as a republican, middle-class person it is not easy to understand why he thought the way he did, but Hoffmann does a very good job in describing the family background and especially the influence of the cult-like George circle on Claus and Berthold. Definitely not hagiographic, and a good read to boot.
Edited to add: New edition, 2007 under the title "Claus Schenk Graf von Stauffenberg. Die Biographie". About 700 pages, so a lot more material.
[not finished yet, but my rating so far:] ( )