A book by John Bellairs that involves a plot to take control of the earth by changing the movements of the Oort Cloud of comets. The scheme is undertaken by an evil scientist/professor type who has stolen a set of carved ivory chess pieces from the British Museum which are being used in a sinister magic ritual. The chess pieces were supposedly Viking artifacts uncovered on one of the islands in the Outer Hebrides.

The protagonists of the story are Bellairs' usuals: Professor Childermass, Johnny, and his friend Fergie.

In the British Museum, there is in fact a set of ivory chess pieces from the Outer Hebrides, which undoubtedly inspired the story.

I read every John Bellairs book as a child (which instilled in me a lifelong passion for Edward Gorey) but this was undoubtedly my favorite, although it's been so long since I read any of them that I can't quite remember why... (There was also another excellent one where the Professor and Johnny go back in time to Constantinople using a refurbished subway but I can't remember the title (if YOU do, please /msg me!)). On a family vacation to London when I was seven, I was considerably more excited to see the chessmen than the Rosetta Stone.