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.
I used this book as an online course in memrise.com: https://www.memrise.com/course/147756... This book consists of many useful and practical words with synonyms and definitions which is essential for people who are taking the TOEFL test. In addition, the words are absolutely useful for improving English language skills in any other scale and are highly recommended to memorize.