An open ending can be such a beautiful idea when an author wants to create a certain thoughtful atmosphere, and to raise several philosophical questions in the readers mind.
However I do not consider such an ending suitable for a children's book series, consisting of 13 books all revolving around the same mystery and the same perpetual questions. The thing is, that my biggest motivation for finishing the series was to get the correct answer and solution. And as a result, I almost feel like I've been cheated.
When that's said, the morale of these books really is worth some serious consideration, and even though I feel the philosophical tone is rather misplaced, it is an ambitious take on the typical childhood writing.