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 was one of two assigned books for my first semester Arabic 101 course. Overall, I felt this book was perfectly structured for helping me to understand a language that 3 months ago I couldn't even pronounce, but now can write just about anything in.
The only thing I didn't enjoy was the companion DVD. I could not understand half what anyone was saying because they were speaking fast and not very clear. I ended up abandoning the DVD's oral lessons entirely about a month into the semester and focusing on just the book.