In what you said, the relationship of Holly and Artemis breaking I find this book was dark. But in everything else TTP was my favourite book. Action all the book, real problems, esporadical jokes, the "Do I or do I not love Holly/Artemis?" and nasty little Artemis gave me all the thrill I was expecting from the series.
I really, in overall, didn't find TTP dark. Like others said, to me the darkest one is TOD, with deaths, trolls, psychotic pixies and angry chefs. The good thing aboit that book though is that Artemis' and Holly's relationship grows A LOT!, but other than that is was as dark as a wolf's mouth.
We can take a good thing of Artemis' lie. He was actually VERY guilty of lyieng to her. That's something good in my opinion