This book is really two separate adventures, with the New York part having nothing to do with the main story. I liked the really creepy villain in this part of the story, I wish there could have been more about him, he would have made a good main villain. The first half of the book was excellent, good action and a good rescue of Grant. The book moves on eventually to the main part of the book, which is really a sequel to the third book in the series called Savage Sun.
Fand and Morrigan return as Kane and crew venture to England to find out why the priory of awen needs their help. Lord Strongbow, posing as Merlin this time also returns. His role in the book is very limited however. Most of Kane's crew spend time traveling around and investigating ancient caverns as they hunt for Strongbow at his base. The scenery and the descriptions of their surroundings are more of a focus rather than action, but these visuals are as usual very interesting and amazing. The conclusion is cool, however brief it was, and although not stacking up to most endings of books in this series, the whole book was another outstanding sci-fi adventure. Like Savage Sun, a lot of legends and lore of the Celtic world are a big focus in the storyline. I liked how these myths and legends were tied in to this book.
There is no mention of the major events in the grand arc conclusion regarding the fate of the Overlords. This book could just as easily have taken place just prior to Dark Goddess as afterwords. In the series as a whole, Grailstone Gambit is entirely independent as far as continuity goes, and is really a filler novel, but nevertheless a fun adventure well worth reading.