I am currently reading book five of 'The Dark Tower' series and I absolutely love it. I think so far, my favourite books within 'The Dark Tower' are 'Drawing of the Three' and 'Wizard and Glass', but I love the way that each book has a different feel to it. Most of the time I don't like it when authors mix genres, but I think the way in which this series combines, sci-fi, adventure and westerns together works very well.
After reading the first novel, I wasn't sure whether this was a series I would continue, but I'm so glad that I didn't, because this just gets better and better.