It seems to me that in an ideal world there would be two types. The first would be for readers who have yet to read the book and the second for those who had already read it.
A review for those who have yet to read the book
The most serious problem here is that the reviewer gives away too much detail concerning the plot. This is especially damaging with crime books, where the reader does not want to be told – by accident or design – who the murderer is, or that the police inspector is actually the illegitimate son of Lord Lucan and Madonna.
With crime books the custom is to warn the reader of your review with a ‘spoiler alert’ if there is any danger of this happening. But giving away too much detail can ruin the reader’s pleasure in other genres too, and there spoiler alerts are uncommon. So if you can’t give away much detail, what do you include?
One solution is to make it less of a review than an appreciation. It will contain more by way of generalities than specifics, giving the reader an idea of the book’s qualities as you see them.
Here is an example of what I mean. It is an appreciation by Hilary Custance Green of John William’s novel ‘Stoner’. It could hardly be done better.
A review for those who have read the book
In this case we can safely refer to specifics. The apparently happy couple have a parting of the ways, leading to an acrimonious divorce and legal proceedings to determine which of them gets the Schnauzer. We examine how well this handled, how convincingly it is done, and whether animals upstage people in books as well as plays.
A review of this type would contain more than assertions about the book but references to it to back those assertions up. It might also make use of quotations, the best way to give us direct experience of the author’s work. It would tend towards what used to be called practical criticism