First of all, can I make something clear to the negative reviewers (and others): this is NOT a Da Vinci Code clone. What Martin Langfield has written is a fast-paced, intelligent, supernatural thriller, predominantly set in New York with flashbacks to Cambridge and London. It weaves occult philosophy and esoteric religious practices into the narrative of a tightly written novel that will keep you in suspense, forever guessing what will happen next.
The so-called "puzzles" which people have called into question are not meant to be on the same level as the Times cryptic crossword, and they work best when you just accept them for what they are and read on to see them revealed. In any case, the climax of the novel explains why the puzzles themselves are not painfully difficult to unravel; they aren't meant to be.
Langfield successfully blends occultism, history, religion, mythology and a commendable grasp of Manhattan's architecture and geography to produce something very different from the average "race against time to stop the serial killer" plot. Indeed, the ambiguities and continual upsets that run throughout the novel often make it difficult to know whose side you, the reader, are meant to be on from one moment to the next. This all adds up to what makes it so compelling. I read the book over four evenings, but agree with other (positive) reviewers that it would work best if read in one sitting.
For those who claim to have found it "difficult" or even "boring", I can only wonder if they approached the novel with the wrong expectations. Forget how the book may have been marketed, this is not the Da Vinci Code. And it's all the better for it.