Another old topic I think is worth bringing back...(I forgot I'd commented on this. X3 But I notice I didn't really address the initial topic question, lol.)
On the environmental issue: I've read lots of stories (particularly short stories) that seem to demonstrate a clear environmentalist agenda, and while I don't think it's at all bad for authors to try to find ways in their stories to promote awareness of the environment, when the agenda becomes too forced or obvious, I think it's effectiveness of getting the audience to think on it goes down. When people feel like they're being talked down to or you might say 'preached at,' I think there's a tendency to get defensive and shut off.
Personally, although I've seen stories where it feels forced (or the story has been created to feed a political agenda, rather than the story simply dealing with those aspects in a natural way), I always thought the issue of the environment felt natural in the AF series, maybe because in folklore, fairies are often associated with nature. And I think it makes sense for a series like Artemis Fowl, where environmentalism has been integrated in as one of its central themes from the beginning, for the series to come to slowly focus more on those themes. TTP brings to the forefront many other aspects of the book that were in the background before, and that progression is one of the things I like about the series.
On the other hand, in TLG,
But anyway, to go back to the original topic of this thread.
From what I've been reading, seems like a lot of people felt TOD and TLC had a darker feel than TTP, largely because of character death. In TOD, Root died, and in TLC, Holly, Qwan and No1 died, and it was uncertain whether they might really stay dead. I admit, reading TLC I had a similar reaction. In all the near-death scenes in AF, from what I remember, Colfer never lets you think the character has actually died—in any scene where one character might think another character has died, he generally puts the scene that shows how they escape right before it. So in TLC when Holly suddenly gets stabbed, and the line, '...leaving Holly to die on the ground, which she did' was a shock for me, and I really wondered if she was gone. It was a very powerful, dark moment, which made TLC one of my favorite books of the series.
However, that being said, TTP had the darkest feel to me. I agree that the characters' physical danger (that is, the feeling the characters might really die) was less strong than in TLC and some of the other books, but for me, the emotional tension surrounding Artemis's lie to Holly made the book more oppressive. I guess it largely depends on your own personal reaction as a reader, but for me, maybe because I see characters under threat of death all the time, often it's situations where a good character turns bad or does something horrible that really strike me and can incite more of an emotional response. I didn't believe that Holly and Artemis would die, but there was a point in the book where I wasn't sure if they'd ever really be friends again, at least the same way they had been growing to be every since TLC. And in some odd way, that sense of oppression and fear was stronger than the fear they might die, at least for me.
Of course, now that I've read the series clear through and know what happens (how tensions are resolved and which characters survive, which is most of them), I don't think it has nearly the same dark feel for me that it did when I was reading the series for the first time. But I think that's natural, and there are still plenty of lingering questions surrounding the themes and character development in TTP to mull over.