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.
First of all, full disclosure: I was a collaborator and contributor to this book, helping Susan Meiselas find people with photos and stories and contributing the essay "Stealing a Good Name" about my youth in the Baliem with the Dani. Susan has assembled a rare and remarkable collection of visual materials about the Dani, beginning with their "discovery" in 1938 by the Archbold expedition of the American Museum of Natural History. (My book about this expedition is forthcoming from University of Hawaii Press in 2017 as "Darwin's Seaplane Pilot.")