For a reader, a book review is often a thank-you to an author, a note telling them how much they appreciate their book. For an author, reviews can make or break sales of the next book in a series. An author's income can live or die on how many good book reviews they receive in a month. If you've written a good book you'll probably have some organic reviews just from readers enjoying your words and wanting to share them with others, but if you want to get into any premium marketing like Bookbub or any of the other large sites, you'll have to have a large number of reviews in the first few weeks after you publish. Other than begging all of your friends online, what's an author to do?
It's All About the ARCs
Paying people to review your book is both unethical and against every publishing site's rules, but a time honored way to get someone to review your book is to give them a copy for free. ARCs, or Advanced Review Copies, are books that are made especially to be given to people in exchange for an honest review. There's no guarantee that you'll get a five-star review just because someone has gotten your book for free. In fact, part of the agreement is that the reviewer is free to post whatever she wants, as long as it's an honest opinion held after reading the book. There are ways to push the odds in your favor, though, through two different ways of finding review readers for your books.
The Time-Consuming Free Method
Readers who like books in your genre are more likely to want to read and enjoy your book. Readers who leave good reviews on multiple books in your genre are more likely to do the same for you. If you take these two facts and add in some time and effort, you'll have a method of gathering readers who want your book and who are likely to give you a review. Begin by looking at the reviews in the top 25 books in your genre. Look especially at books with multiple reviews. Find readers who do frequent reviews, and make a list of them, along with their email addresses.
Once you have this list, email each reviewer. Mention that you saw their review for X book, and that you're about to publish one in the same genre. Give them a one-sentence description of the book. Ask them if they'd like a free copy of the book in exchange for an honest review. Most frequent book reviewers are used to requests like this, and they'll honestly let you know if they have the time or desire to give you a review. Email copies of the book to everyone who agrees to review it, and send them a link to the review page once your book is online and live.
The Quicker, Slightly More Expensive Method
If you'd rather spend your time writing your next book instead of searching down names to review this one, there's a service that will do all the searching for you. BookRazor is a service that does the searching for you, choosing only reviewers that read and review in your genre. They charge $25 for 50 names, which can take days for amateurs to find, so unless you're completely lacking in funds they're a smart service to use.
Keep the Faith and May The Force be with You!