Address your letter to the person in charge of doing the reviews when possible. On websites, this information is typically listed. If you have no way of finding out to whom to address the letter, you can address it to "Book Reviews" or "Fiction Reviews," whichever is more appropriate. In the upper-left corner, write the name and address of the company you're sending your letter to. You can skip this part if you're sending a request by email. Magazines and websites that specialize in your type of book are perfect targets.
Introduce yourself in the first paragraph and give your full name. Let the recipient of the letter know that you are an author of a book you would like to have reviewed. You can add a brief description of your publishing history, but stick to the basics. You aren't trying to sell yourself as a writer. You want to get your latest book reviewed. Give the title of the book and the number of pages involved.
Write a brief three- to five-line synopsis of the book you would like to have reviewed. State that you would be happy to send a copy of the book for review. Close the letter by thanking the reviewer for his time and stating that you look forward to sending a complimentary copy of your book for review purposes.