Not an ebook, and the books are not free, but the full
Schlock Mercenary archives can be found on the web.
I mention this because my 9-year-old was an incredibly reluctant reader until he discovered Schlock. I don't care if they're comic books with mass mayhem and few words per page -- he READS them. I almost cried the first time I walked past his room and saw him curled on his bed, nose-in-book, giggling to himself.