Decide what book(s) you would like to review. Once you decide, you need to track down the publisher information, which can be easily done by searching for the book title on the Internet. The book description provides the publisher's name, as well as the ISBN-10 number.
Contact the publisher. Their contact information should readily available on their website in the 'Contact' or 'FAQ' section, or found through a simple internet search.
Write a letter to the publisher, addressing it to the contact person listed, if there is one. The letter should be concise and include: Book names, ISBN-10 number, why your readers would be interested in this book and a little bit of background on your website or blog. Typically, the publisher will contact you within a few days if they are interested in working with you or if they require further information.