Sequels to classic books

Well, I was on amazon.com the other day, searching for les miserables merchandise, when I found a book heralded as Les Miserables 2, known as cosette, or a time of illusions. It Is supposedly the story of Marius and cosette after Valjeans passing, in a world here Thenardier stays in France and saves javerts life. It is also a sequel to the musical instead of the book, and was written in the 1980's, obviously by a different author. Is the book good? I don't know, but most reviews deny it, but I plan to read it anyway. I understand that due to the books age, it is now without copywrite and anyone can make a sequel or prequel or whatever they like, but do you think it's fair for someone to add to a classic series like this? I don't, but legally it can't be helped....and at least it's not recognised by fans, but I don't think sequels like this should be able to bare the name of the franchise. It's basically piggy backing the series for advertising. but what do you think?