I absolutely loved the HBO series, which I why I started reading the book. I think the TV series actually turned out better than the book, just because you can do so much more visually and you can tone down the inner monologue a bit. But because the series deviated away from Lindsay's continuing plot, the books are an enjoyable read in and of themselves. The written character isn't as lovable as Michael Hall's portrayal, but I still find myself rooting for Dexter in the end.
But I'm not here to quibble over differences.
Dexter in the Dark is my favorite so far (but I still got two more to go). I liked it for its unique angle, not only as it concerns Dexter, but what it has to say for psychopaths everywhere. With every book I find myself more intrigued and biting my fingernails wondering how Dexter will ever get the best of his new subject.