Hmm. I might give this one 3.5 stars if the option existed, but perhaps not. My reason for considering doing so is admittedly perhaps a bit trivial. According to the genre identification on the book cover, this is "Science Fiction." However, it is in fact clearly fantasy, a work in which magic and spells are explicitly invoked as the agencies whereby the fantastic occurs--in this case, the ability of some folk to travel between parallel worlds. There is of course nothing wrong with fantasy, but I prefer to know that that's what I'm reading up front.
That said, this is an engaging and well-written (unsurprisingly; Robert Charles Wilson has a stunning facility with language) adventure story. Admittedly, I was somewhat bothered by some of the character flaws--characters who tenaciously refuse to face reality because they don't like it (in this case, the mother who refuses to teach her son anything about his unique ability, despite KNOWING that there's a dangerous "Gray Man" out there who might track him down, because she wants her son to have a "normal" life) bug me. I don't find them sympathetic, or even particularly easy to understand. Nevertheless, the characters are generally well-developed, with satisfactory blendings of traits that make it difficult to view even the villains simply as bad guys. They are generally given plausible motivations, especially Cardinal Palestrina, who must confront the moral implications of abusing people in order to achieve victory in a war. Do the ends justify the means? Fantastical fiction can provide a useful context for exploring such questions freed from the commplicating factors of real-world politics and ideology. Wilson's recurring interest in alternate worlds and the implications of their existence is of course also strongly in evidence (I haven't read all of his books, but several others I have read address similar ideas).
Anyway, if you like well-written, tightly-constructed parallel worlds fantasy, this is a very good book.