Book reviews are interesting for people who don't have the book but may be interested in it. Rich people can buy the book to find out hwat it is all about. Most people don't have the resource to just do that so they try to find a review to find out wheter the book is interesting or not.
This is extra important for technical books. If for example you are a good programmer you will probably not be interested in a book for beginners. Well, the title isn't always clear. "C programming for beginners" is relatively clear, as is "Advances multidimential matrix programming in Fortran04". But is "C++ programming on the Altair" a book for beginners or for advanced programmers? Either you run to the book store, grab the book (suposing they hav it on display) and read some random pages (often the back-cover is a good indication) or you find out from someone who has used the book. Via a review.
Review writing is not simple. Just throwing a string of Wows and Hos and Greats isn't going to help anybody. Neither is it helpfull to just say it's a bad book without giving a reason.
For a technical book you can easily keep yourself to straight facts and maybe copying a few snips or even a full page (ask the author for permission first, for such snips and a few pages they will probably happy to provide and maybe even helpfull at sugesting some sections). It's most helpfull too to indicate how the book has helped/hindered you.
Reviewing a Romance or detective story (or most non-technical books) it's important that you show the plot without ... revieling the plot. Which can be a bit tricky. Remeber that a review is not intended to be a full (shortened) recount of the book so people don't have to read the actual book to know what's in it. The intention is to present a taste of the athmosphere and style, pionting out a rough overview of the plost-directoon. Read a lot of back-covers and other reviews before you start posting your reviews.