I'm actually glad I just recently discovered this trilogy because frankly, I'm happier when I don't need to wait a year for the next book to come out.
Circle of Fire is the last installment in the series. Lia, the Angel of the Gate is out looking for the Stone and the last remaining key. She sets out on a journey to Ireland based on a researcher's discovery on what the passage means, and to banish Samael forever she also needs Alice's help. That part is the hardest.
The first few chapters are almost agonizingly slow. I fell asleep at one point, during her journey to Ireland with Dimitri where she meets Maeve, and I don't ever fall asleep when reading. However, it pays off after that. There are so many new characters that arouse suspicion, and there are enough twists and turns to keep us guessing. The outcome is also quite satisfactory.
One thing I'm not happy about is Lia's character. She has grown from a passive character to one so prickly and full of distrust that I find her slightly unlikeable. Of course she's strong and she grows even more so after the challenges she faces and the difficulties she endures, but that demeanor is making me feel she's an entirely different person.
I also don't feel that the final outcome is properly heartfelt and satisfactory. I want to know what happens when Lia travels to the Plane and meets Samael. I want to know what happens outside of that, and the trigger behind 'the big surprise'. I want to know more about the lives of the others after everything ends. I wish we could have an epilogue of some sort.
I also wish we get to see more of Lia's actual power - one as a Spellcaster, instead of the scrying talent that is being quite repetitive in the book. I want to see her fight, and strong, and for the Sisters to lend her power (like that one hair rising chapter with the Sisters protecting Lia when she supposedly 'dreams').
That being said, I'm happy enough with everything and I'm sad to see it end ;)