Does An Ending Have to End?

Call me old-fashioned. Traditional even. But I think every book should have an ending. I'm not saying every thread has to be tied up or even that the reader shouldn't be allowed to choose their own resolution. What I am talking about is series books where at the end of a book, the plot is left wide open in an attempt to ensure the reader doesn't stop at that point. To me, there's more marketing than storytelling in that.
Just before Christmas I finished The Keeper of the Grail , the first...