The fifth in a series, it's a great book even for older people, as long as it's being read for fun, because that's what it is, a great fun, exciting book. There are new developments (some surprising, some not) and suspense.
This book had parts that seemed a bit unnecessary to me. There were parts that I wondered why they were so long or why they existed at all because not much (or nothing) happened. So, in that aspect, it was a bit slower than I'd expected it to be. It could've been much shorter and not any less interesting, but I guess she cared about the word count.
There are plenty of mild peril situations but no more than the other books. Nothing too graphic.

Spoiler

people losing control of their minds, burns, mention of Sophie's kidnapping and being burnt and they visit the site, unplanned attacks, parents betraying their children, explosions, etc.

There are great friendships going on parents who genuinely care for their children and don't want them hurt.
The flirting is upped and something is mentioned almost every other paragraph. It's mostly between Sophie and... I'll leave it as a surprise so I don't ruin it. (But there are two of them. They're jealous of each other.) Also between quite a few other characters. Lots of talk about boys, holding hands, hugging.

Spoiler

They almost kiss,

she cries on one's shoulder. There's a sleepover with two boys and two girls sleeping in the same room. And of course, as in the others, there's lots of a girl falling back with a handsome boy catching her.
And the whole "elves are superior" theme is mentioned once or twice. If they were real, I guess they'd have the right to think that, though.
All in all it's a great clean book that I'd totally recommend. To anyone. (Unless you don't really want to buy a series that'll probably have at least seven books and can't borrow it....)
Oh, and good news! There's sort of a cliffhanger

Spoiler

and a horrible death

but it's way more bearable than last time.