The final battle takes up a third of the book - magic, doubt, evil, and plenty of cool sequences. The geography of the island is exploited well, these familiar places being used by the islanders to boost their defences. Plus some "key" locations being used as the centre of great action pieces, particularly the lighthouse.
Being a finale, it wouldn't be complete without a big bad and some serious self doubt. Morrigan's ability to steal people's souls and make them into puppets is pretty unnerving, and her brothers are the big muscles. After losing his grandfather in the last book and realising the final battle is nigh, Fionn naturally is going through the dark night of the soul. To be fair, I completely understand the wariness of everyone around him - if I knew I was about to be caught up in a fight, I would not be enthusiastic about a 12 year old being put in charge, regardless of how much magic he had (even if he could initially control it).
I liked how the tendrils of myths and clues laid back in the first book play out in this one, everything wrapped up into a neat package (though there is a bit of "don't think too hard about the linearity of time" going on. Still, the candle magic means the time travel aspect feels less like sci-fi time travel, and so loses the shaky logic vibe).
The images above the chapters are really cute, and really clever. They're all set in exactly the same location, but they alter with the events of the book, and also track the time of day too. They're not large and you'd need to be paying a lot of attention to them to notice the subtle (and some not so subtle) changes. It's something the publisher could have easily left out, as it would have been a fair bit of work for the designer to create every one, double crossing against the story, but it's such a lovely touch.