Books can be attributed to "Unknown" when the author or editor (as applicable) is not known and cannot be discovered. If at all possible, list at least one actual author or editor for a book instead of using "Unknown".
Books whose authorship is purposefully withheld should be attributed instead to Anonymous.
This is THE BEST vegetarian cookbook. It might be one of the best cookbooks generally. This book is so ahead of it's time and it remains completely unsurpassed. Every single recipe is excellent, but the reason and purpose behind them is also. I recommend that you learn German to cook vegetarian, just as people learn French to cook fish.