Nightmare Fuel: There is a civilization of advanced formerly mythical creatures living in the center of the Earth that have the ability to watch anything and everything we humans do. They can also turn invisible, mesmerize people, and they wield lasers.
They're also able to cut you off from the rest of the world with time stops, make people drop dead for no explainable reason with bio-bombs (you know how people drop dead from terror? Maybe it was the doing of the Fair Folk), and the majority of them view humans as inferior, if not outright okay to kill if it wasn't for the Masquerade.
Anvilicious: Book 6: Go green, and you'll save the krakens, stop poisoning fairies with Spelltrophy, and keep the world from dying. Plus, Artemis Sr. puts emission filters on every vehicle they own, including the jet and helicopter. Though the series has dropped environmental anvils all over the place since its beginning.
Ass Pull: Eoin loves this trope or owes it money. Technology of the series doesn't appear to have any consistency: numerous vital plot points happened just because fairy technology happens to be capable to do something we weren't informed of or has a limitation never brought up before.
One short story in the companion book is set in the period between the first and the second book, aka when Artemis was still a egoistic criminal mastermind and Mulch was believed to be dead by the LEP. Holly goes to stop Mulch in his tracks, but she isn't supposed to know Mulch is even alive... Well, let's make him wear a mask! Voice? Did we mention all the dwarves' voices are exactly the same and even the dwarves themselves can't tell one from another? Because now they are.
Didn't Foaly tell Holly that he thought Mulch was still alive, though?
Fridge Brilliance: A page-long dialogue sequence between Holly and Artemis midway through the second book where Holly asks why Artemis is so cold-hearted when his father was more noble. His answer was that he "made a mistake." Said mistake is explored in-depth in the sixth book.
Fridge Horror: Artemis never takes out his iris cam while showering in EC. This either (1)Fuels the fire for the Holly-Artemis ship as it says she's watching through the iris cam, or (2)Ho Yay between Foaly and Artemis, which has been mentioned by certain people.