O is for ordinary, and when I say that, it is not a comment about the quality of this fine book. It rather relates to the way in which daily life, the quotidian, the normal, is used as a setting for a story which, on the spiritual level, is a roller-coaster ride of danger and excitement.
The danger portrayed in this book is one which every settled Christian may face. Probably devout people in other religions run the same risks, too - almost certainly, I should think. It is the danger of becoming smug, of stopping living on the razor's edge and falling into complacency. This is one of the most serious spiritual dangers there is, as many writers of devotional literature have pointed out, most notably C.S. Lewis in The Screwtape Letters. Smugness and complacency in religion lead one into a deadened state in which acts of the most shocking wickedness can be committed, without even a faint awareness that one is sinning, and this is, in fact, what happens to the protagonist of The Christmas Dog. Happily, it all comes right in the end, but we are left with a dizzying sense of the protagonist's narrow escape.
A valuable cautionary tale for any Christian. Or follower of other religions.