I love this book series. It has a great cast of fun characters and is full of great laughs but also has a serious undertone. He also is great at laying the ground work for latter pay offs. I mean I am pretty sure the whole character ark is summed up in the title of the book alone. If you do not know what I am talking about here is the quote.
Friedrich Nietzsche - Whoever fights monsters should see to it that in the process he does not become a monster. And if you gaze long enough into an abyss, the abyss will gaze back into you.