A lot of websites review books, either officially or unofficially. Good book reviews tell what the book was about and whether the author did his or her job in delivering a good book.
If you want to make the reviews you write on your website stand out from the crowd, try doing more than posting a couple of sentences about the book's storyline.
Offer a synopsis of the book you're reviewing. Not every site gives synopses so if you do, that will make your reviews and your site special. Of course, you shouldn't reveal story aspects that will ruin the book for readers. Nothing's worse than looking forward to reading a book, only to have it spoiled by some detail-oriented reviewer.
Sure, it's challenging to write an accurate synopsis and to express what's good about a book without leaking the plot twists and character surprises, but it can be done. Be sure to give your honest, but fair, opinion also.
Don't be afraid of writing book reviews. You're doing a service for the author who needs readers.