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.
While this is a very helpful guide on how to create a nice looking home with lots of handy little tips and ideas for making rooms more useful while maintaining the aesthetics, for me it did lack a certain amount about personal touches and how to really make a place 'your' home. This is no fault of the authors, more just my preferance for homes to be real homes and not show homes designed to impress other people. Still a handy book though and one I suspect I'll be refering back to when I get a bit of decorators block.