The Belgariad by David Eddings is a great example and one of my all-time favorite series. Every book ending made me want more, and when the first series was over I was saddened. I loved the characters, and was hungry to know what happened next. After reading Eddings' books several more times I was awarded with his sequel series The Malloreon. It was similar to The Belgariad like Shakespeare's Henry IV part one is similar to part two - they are practically the same story but I didn't complain; I considered it a gift. If the book ending is good enough, I'll pick up the sequel. If the series is good enough, I'll read it again.
The last dozen chapters of my first fantasy novel, Angst, was a flurry of furious writing. I was so caught up in the events of the story that I couldn't stop. I knew the ending, but I wanted to know how they got there - so I kept going. I also knew that, while there could be a sequel, there didn't have to be. Angst could stand alone, even though I included several things that would lead into more books.
A good stand alone novel wraps up all of those loose threads and provides you with a sense of completion. You've been with the heroes throughout the story, and should reap the rewards a good novel provides. When reading a series, the plot threads and amazing characters should make you hunger for more. Throw in a cliffhanger and you should be desperate for the next novel. The last book in the series should be so satisfying, that the hours you spent on the journey are worth it.
As I approach the end of Buried in Angst, I find myself being more cautious (for now). This is the second book, but not the last, which changes my goals in how the story ends. I want to wrap up this novel in a way that everyone feels a sense of completion after reading it. At the same time I have to continue growing the world of Ehrde and the challenges Angst and his friends will face. My intent is not closure, but that the end of this book will pull readers into the third, while providing story arcs that will entertain until the last.
Oh, and for the record, there will be a final book. Like The Belgariad, Angst has an ending, and I know what it is. It will be fun, and exciting, and, I hope, make you hungry for more.