Each character is written brilliantly, even if I do find some characters like Hekate changeable.* Dee and Flamel are written perfectly, stealing the story from our two main characters. If anything though, the greatest character is Perenelle. The way she is written is very entertaining, and the pure will and determination of Flamel's wife is the key principal to her realistic characterisations.
The plot is simple, too simple perhaps, and could be simplified to this: Josh and Sophie stumble across Perenelle being kidnapped and, being children from a mysterious prophecy, are taken by Flamel to Scathach who suggest she goes with them to Hekates house. Arriving there, they start to receive their magic, but have to leave in a hurry because of a battle. They seek out the witch of Endor for assistant and then fight zombies. The book ends there. You may not think of that being fair, but trust me, once you've read it, you'll think it is. The book is brilliantly written, yes, but the plot, despite being interesting and fun, doesn't really grab the imagination in a way that makes you desperate to read more. I, personally, have a rule, if you don't want to read the next immediately, it isn't worth reading. Obviously some books are exceptions, such as James Bond, Sherlock Holmes and Charlie Bone, but this isn't. Despite a mild curiosity as to what Scott has in store for the Newman twins, I'm not overly motivated to read further into this world.
*that's a joke you'll only get if you've read the book, by the way.