Ending Spoilers are used in this revue.
This is one of my favorite series after similar genre types like Harry Potter or Discworld. The premise for the series is original, a modern day cops and robbers heist caper with fantasy elements. To this end I enjoy the first four books of the series. That could have been a closer for me.
Let's recap the endings of each book:
Book 1, Artemis schemes the fairies of their gold and gets his mother's sanity restored.
Book 2, Artemis and the LEP develop mutual respect, save Haven and find Artemis Fowl the Senior
Book 3, The LEP is forced to mind wipe Artemis and the Butlers but Artemis has a memory gambit in store, tying to the next book.
Book 4, Haven is saved from being revealed to humans, Holly quits the LEP because of Root's death and Artemis and Butler get their memories back.
Book 5, Artemis, Holly, and N1 are sucked into Hybras and help move the island out of Limbo. Unfortunately there's a time skip and everyone else has been waiting for them for two years.
Book 6, Artemis reveals the existence of fairies to his mother and pushes Past Opal Koboi to the edge but she is still stuck in the future to cause more time paradoxes.
Book 7, Turnball Root calls off his scheme due to pleas from his dying wife, making the efforts of our heroes almost pointless. Artemis still has his mental disorder and the ending sentence has Butler stepping in turnip soup. I am not kidding.
Here's what I think causes the derailment of this series: when young criminal mastermind Artemis Fowl stops being a criminal, he stops being a mastermind. This starts after Book 5 that ends with the time skip, especially jarring given Butler's rapid aging in book 3. Artemis handled himself fine in Book 5 until they got to Hybras. In Book 6, he travels back in time and is suddenly getting outsmarted for most of the book. True, his memories aren't really what he thinks they are due to Time Paradoxes but come on, this is Artemis Fowl we're talking about. He should be more competent. His only bits of brilliant strategy in this book are bringing his past self to the present and tricking Koboi in the chase. This becomes worse in Book 7 given how the characters affect nothing, leaving a downer ending to the next book that will hopefully wrap up the series.