My friend and former teacher gave me this advice about writing book reviews. It was so good I've saved it:
"The best criticism is essentially appreciative; it points out what is good in somebody. One reason I find Virginia Woolf such an illuminating critic is that her approach to writers is extract the proverbial wheat from the chaff. And this from someone who could be catty and cuttingly witty at somebody else's expense!
I always think it's a good idea to be specific, of course. Avoid vapid generalization. If you make a larger point, work it from specific exploration of a passage or chapter. If you can find representative passages or if you can find passages that matter to you, I'd concentrate on those. You can say a good deal about the whole book by zeroing in on specifics, for all writers have ideas that absorb them. That will also help you from sprawling, from trying to do every aspect of the book justice. It just isn't possible.
And, finallly, one of my favorite refrains: use lots of verbs and nouns. They give any review punch."
That's quite general advice from him, of course. As for a set formula or format. . .there probably is one, and I probably follow it subconsciously, but I don't know what it is. I tend to start off by saying something that remarks on a writer's past book or books (if they've written another), something like, "Author so-and-so has, miraculously, exceeded the promise of Book A , her acclaimed first novel/collection." Or "So-and-so has put out a fourth collection, and thislike his othersis solid and sure of itself." (I usually write about poetry collections for pretentious journals, so there's no reason for your language to be as stuffy as mine.)
I point out the good things about the book, using lots of specific examples. I might point out the themes, or style-- again, backing up what I say. If I think a book is crap, I'm afraid I usually say so. . .despite my teacher's advice to the contrary. I think if something is really bad and you're not honest about it, you end up losing some of your credibility. But it's rare to get something awful.
I'll write to my friend who does book reviews constantly for Publisher's Weekly in the US, and see if she uses a formula of some kind.