At first, I thought the book was a little bothersome to get through; the twin brothers (new additions) were unnecessary and their characters weren't great. They seemed a little like they were intended to be a little too much like Artemis. That's okay, but it seems unnecessary if one is to consider that they might grow up to be just like him and you have three of the exact same character.
During the beginning, I also felt like the magic he'd stolen was misused. At the end of the last book, I'd hoped there would be more to his magic than an attempt to heal his mother. I had hoped he's still be trying to steal stuff (or something) and using just a little bit of magic to help him. Actually, having thought of it, I hope he does regain a little bit of magic in future (I would like to read more of that type of thing).
Number 1 was not misused, in my mind. I like the way he is trying to aid the People's society; lifting old curses and aiding the policing of the underworld for the better. I am also glad to see that he is still in "training" for his magic. It seems so much better that he isn't "all-powerful" and that he still needs to learn, but is doing the best he can. The limitations on his magic are quite interesting as well and provided a reasonable time frame to the piece; three days back in time, the ability to bring people forward in time from their prior locations and even have people go back to leave minor clues. It seemed reasonable and did not overstep the bounds of my belief as most time travel novels and television shows do.
I thought the miniature feud between Artemis and Holly was unnecessary. The way Mr. Colfer had written about the death of Holly's mother would have seemed like a better reason for her travelling back in time, rather than the mistreatment of magic by Artermis; that she was trying to prevent his mother dying in some vague way of trying to understand her own mother's death. Though, I did like the reappearance of Commander Root to try and make it up to her.
The created paradox was amusing and certainly explained Artemis' supposed sudden interest in fairies before the start of the first book. Though, my main wonder is: when he was mind wiped between books three and four (I believe it was), and then he regained his memories... Surely at one point or other, it seems logical that he would remember the events of the extinctionists and his time travel to the future.
The majority of the way through, I couldn't help but wonder how much the younger Artemis would find out about fairies. I realised that he would, presumably because he'd forgotten and it was understandable considering he would likely be mind wiped. However, I did not (at all) see the inclusion of Opal Koboi and was somewhat surprised at he introduction to the novel.
In the end, everything slotted together well. The use of the stuffed monkey, the use of his magic, the kraken and even the prior mentions of Opal's use of the lemurs.
Overall, the start of the book was a little unamusing and I was less than impressed with the use of Artemis' little brothers; they were a useless addition to the plot and the characters don't really fit in, but that is my opinion. Maybe Mr. Colfer will have a larger use for them later on.
-However, I did like the use of the toy monkey. Though, I believe it could have still been relevant if he didn't really introduce the twins at all and just had Artemis making the toy or picking it up after one of his brothers. I understand the humanitarian Artemis is trying to become (creating a better world for both humans and the People), but the displays toward his brothers was just a little bit pretentious. (Teaching his brothers restaurant ettiquette? A little over the top...)
I wanted to read more of Minerva, the girl genius, but it seems that she is not relevant within the novel. Hopefully she will make a reappearance later.
I like the reappearance of Opal (of the past) and can only wonder what Mr. Colfer has in store for her. I assume that she cannot die in this timeline as she (logically) has to go back to her own timeline and live there as well. Her pixie twin companions were amusing as well and they are certainly good characters.
As mentioned above, I would certainly like a bit more explanation of Artemis' magic. Well, not an explanation, but I think another appearance of trying to gain magic would be interesting. Maybe Number 1 could help.
The time travel aspect has been correctly done, bother times and I did like the changes in the time tunnel where Artemis became older, Holly younger and even how Opal could emerge a few days prior to events (seemingly unnoticed by Number 1).
I though the book wasn't as good as the prior novels in the series, I still think it is better than a three star rating, though not perfect and would not rate it as five star. Four seems fitting; despite a few unnecessary characters (the twins, the mesmerised doctor) and a few scenes that I thought gratuitous, I think it was better than an average novel.