I have really mixed feelings about this book. First of all, I saw the movie of I Am Number Four a month or two ago, long before I read the book, and both my husband and I really liked the movie. So I was surprised when I found I didn't like the book as much.
Part of it is the writing style. I think it's crap. I'm not usually too fond of first-person present-tense, for one thing, but I also really hate it when authors use incomplete sentences for effect (or so they think). I Am Number Four is full of that kind of thing, especially during the action scenes. The result was that I had a really hard time getting into the scenes that should have been the most tense and compelling in the entire book.
The book's plot was also a little different than the movie's, and in most cases I actually found that the way they did it in the movie seemed to work better.
I don't know whether knowing the author's identity made a difference in my enjoyment or not, but I suspect not — the grammar would have bothered me in any case. I just don't think the guy can write. But anyway, the author's name is obviously a pen name, so my husband looked it up — and found that the author is the infamous James Frey. No wonder he used a pen name, with that kind of stigma attached to his name! Of course, he was vain enough to leave "clues" to his identity in the novel. Idiot.
I loved the idea and the story line (which is why I liked the movie so much), and I will probably read the other books, but I definitely enjoyed the movie much more. This is one case where I don't mind advising people to skip the book and just watch the movie — it's much more enjoyable!