Well, Michelle, as with any adaptation of a book to screen, there are big chunks of it left out. With the book, the characters were clearly delineated, but in the series, which is enjoyable nonetheless, you'd need a scorecard to keep track of who's who.
Very good choice of cast, I thought, with the leads being particularly good.
I also enjoyed "Pillars" as as book on CD.
Ian McShane (remember him as Al Swearingen in 'Deadwood'?) is a perfect villain.