When readers leave a review for the book that they have read -- be it on Amazon, Goodreads, or their own blogs -- they spread the word about it. As Tim O'Reilly famously says, "Obscurity is a greater threat to authors than piracy."
Sites like NetGalley offer a great opportunity for authors to find readers and conversely, for readers to discover new authors and books. While publishers/authors have to pay a subscription fee to NetGalley, readers and bloggers can read books for free in exchange for putting up a review. It also takes away the problematic issue of vetting every review request. I seriously wish there was such a site for Indian books.
Here are some ways to engage with readers and reviewers.
Goodreads - Putting up a book on Goodreads and getting marked as "want to read" spreads the word about the book. Those who have physical copies of their books to offer also run giveaways on the site.
Blogger Community - There is a large blogger community who read/review books on their blogs. Sites like Indiblogger offer an opportunity to network with this community.
Social Media - Every author has a web presence, be it a business page on Facebook, Twitter account, Instagram, etc. A sustained campaign ahead of the book release will result in higher visibility for your book. And organising a giveaway through your blog/website or sites like Rafflecopter can generate interest for your book.
Newsletters - Sending out a newsletter is one of the best methods to grow your audience. The risk is that your newsletter may end up in the spam folder.
Sharing other authors' work - Teamwork can get your book greater mileage. Joining hands with other writers and sharing each other's books on your timeline will enable you to access a larger viewership for your book.
A word of caution. Sending out PDF files of your book randomly to reviewers is not a great idea as it's an easy way to get your book pirated. Instead, consider sending out Amazon gift cards to interested reviewers who can then purchase your book and review it. Or else send out .mobi copies.
While every method has its pros and cons, there is no doubt that getting your book reviewed is the only way to find more readers.
Do share your methods of getting your book read and reviewed.