Buzzfeed throws up a real mixed bag of articles, and they aren't always the most in-depth or thought-provoking. But this one from a few months back -- light as it is on details for each entry -- is pretty interesting:

Here Are 25 Book Series That Actually Got Better As They Progressed.

The first book of a series tends to be the one that draws the most readers, as it's the natural starting point for newcomers, and not everyone will progress beyond it. But a really good series improves as it progresses and we get to learn more about the characters and see them in different situations. Indeed, I'd go so far as to say that the success or failure of a series depends more on the last book or two than the first book or two.

I was pleased to see Cirque Du Freak get a mention, though I scratched my head at the comment: "They lost it around Books 7 through 9." REALLY?!? Personally I thought that's when things kicked up a gear! I guess there's no accounting for taste... :-)

https://www.buzzfeed.com/noradominick/book-series-got-better-as-they-went-on?