Some book reviews are very short and others, such as those that are published in The New York Review of Books are usually very long. They can run from two or three hundred words to several thousand. I think people read book reviews to see whether they would like to read the books themselves. So a good approach to writing a book review would be to tell what you yourself would like to know about the book if you hadn't read it.
The main thing a reader wants to know is what the book is about. If it is nonfiction, what is the author's thesis? If it is fiction, what is the essential plot? What is the essential conflict? But don't tell how it's resolved. A book-review reader will probably want to know a little something about the author. This is usually provided on the dust jacket if it is a new book. If you are reviewing an older book, you may have to refer to the Internet or to Contemporary Authors at the library.
The last thing the book-review reader will want to know is your opinion. This is where you need to show restraint. Your reader may find a book very interesting when you personally found it boring. Abraham Lincoln once said something like, "This is a book you will like if you like this kind of book." Or maybe he said: "If you like this kind of book, you will like this book."
The New York Times still publishes a book review section every Sunday (while many other newspapers have stopped reviewing books or cut back sharply). The N.Y. Times sets the standards for book reviewing in this country (as does the London Times in Britain). You should look at a copy of the New York Times Book Review in a Sunday edition to get an idea of what is generally considered a good contemporary book review. (A lot of book reviewers get their opinions from the NYTBR.)
In addition to being informative, the Times reviewers try to make their pieces interesting and entertaining. So they will pick out interesting or unusual facts, comments, opinions, etc., and either quote or paraphrase them.
Books are reviewed in newspapers because they have some news value, since they are new. So if you are reviewing a new book you should ask yourself what news value your book has, or is supposed to have.