The Wind Through the Keyhole by Stephen King

This book is definitely the best in the Dark Tower series. In some respects, it's just a side note, a pause in the story, a place that you wouldn't even necessarily have known was a gap until it is filled in. But it really clarifies the relationship between Roland and his mother, and in its limited scope, it is beautifully done. Well worth reading even if the rest of the series frustrates you. The plotting (with stacked stories) is magnificent, and I really felt the story spoke to the heart of what it means to reach the Dark Tower.