Andrew Roberts provides a great biography but Napoleon The Great is the same as Napoleon: A Life which came much to my displeasure. Buy whatever is cheaper since it's a page for page reproduction and I would venture to say that the title was changed simply for mass production since it's a less Bonapartist title.

There are indeed, many of which are very poor, many of which are little more than hagiography. My personal recommendation would be this one http://www.goodreads.com/book/show/12953773-napoleon as, whatever other book you read, it will give a good balance to it. Also reviewed here http://www.washingtonindependentreviewofbooks.com/bookreview/napoleon-life-legacy-and-image-a-biography and http://www.napoleon.org/en/magazine/interviews/files/480725.asp