Holy Smokes is the fourth in a series, and it's immediately apparent that it very much helps to have read the previous novels. There are a lot of characters and a huge amount of world-building that isn't explained in any depth in this novel, and it's quite hard going for someone new to the series to follow all the different situations, people and powers.
Aisling Grey seems, in a rather Anita Blake manner, to have picked up a number of different skills/roles in the course of this book series. Not only is she a Guardian; she is also a Demon Lord, a Dragon's mate, a Prince of Abaddon, and various other things. She has a voice in her head trying to encourage her to the dark side, a demon familiar that's a dog, and various other assistants who appear at various times. Her love interest, Drake (a dragon), spends a lot of this book disappearing, being kidnapped, and keeping things from Aisling - they are meant to be getting married but things keep getting in the way so that one or other of them doesn't make it to the church. A side note, here, that if you didn't arrive for your wedding in a Church of England church on Saturday, there is no way you could have the wedding the next day instead - things just don't work like that!
Anyway, the wedding is a theme throughout the book as various things crop up that require Aisling and Drake to go off and do some troubleshooting. They travel around a lot, both in Europe and to Abaddon; various old foes reappear, along with some new friends (or are they friends?). The plot has many strands taking place at the same time, and Aisling seems to spend most of her time rushing ahead where angels fear to tread, getting in the way of things that she doesn't know about, and requiring Drake to tell her he loves her. Despite all this, she's an appealing character in spite of her blundering, and her desire to trust people comes across well, if a little naive.
Those who have read this series from the beginning will enjoy this latest episode. For newcomers to this series, this isn't a great book to start with as there is too much backstory to catch up with and too many plot threads to give it an overall shape.