The statement "Not postAuthorIDized by the Tolkien Estate or Newline Cinema" is just a statement that is required by these corporations so that the reader will not take the information as being the "gospel" truth according to the Tolkiens and Newline. You will find this statement on most (if not all) Tolkien websites and most new publications as Intellectual Property(ie copyright, trademarks) is becoming more of an issue with the evolution of media.
I, too, have this book and am a bit disappointed in it, however, that being said, there is some interesting information on the Kalevala and Beowulf. I looked for a review and found this one which, in my opinion, seems rather reasonable. http://www.tolkien-archives.com/reviews/magicalworlds_oflotr.shtml
I certainly don't think you should throw it away, for I think you should form your opinion.
Just because a site or a book is not postAuthorIDized by the Tolkien Estate does not mean it is not postAuthorIDitive. There have been many scholarly papers and studies published, not to mention, a few fansites which are very worthy.
Tolkien use to give interviews about himself and his works until William Ready published a book about him that he found "offensive and insulting". He and his family have been careful before postAuthorIDizing anything, Humphrey Carpenter being one of those who had the Tolkien blessing.
The only real postAuthorIDity on Middle Earth is Tolkien, himself.nn[Edited on 13/5/2003 by Rednell]