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 a really good introduction to sequoias and sequoian ecology. It's focused on the General Sherman Tree (hence the title) and there are some great factoids in it. It was published in the 90's I think so some of the information is not completely up to date, but you would only know that if you were a sequoia nerd. Nice and short, a good souvenier of a trip to Sequoia National Park.