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One style guide I can absolutely recommend is Deutsch für Kenner by Wolf Schneider, a German journalist and language critic. The book is both witty and clearly structured. (Note that it takes a firm stand and is by no means a neutral guide; some even call it arrogant.) Important to know: You'll need a good knowledge of German to really enjoy the book.
If you've already read Deutsch für Profis or Deutsch fürs Leben, then you already saw a lot of the contents of Deutsch für Kenner.