If you list retail links to your own books on your website, and if you feature the books of other authors on your blog, then you may as well get a commission for sending traffic to a book retailer.
Let's start with Amazon. It's easy to set up an affiliate account through Amazon Associates. https://affiliate-program.amazon.com/
Whenever you want to include an Amazon link, simply log into your associate account, click on Links and Banners and type the name of the book or author. Once you find the right book, you can easily copy and paste affiliate link code into your blog or website. Then you will make a commission on any qualifying product that the customer purchases after they enter Amazon from your website.
Sometimes they might buy the book you linked to; in my earnings reports, I've also seen purchases of other books, toys, DVDs, videos, Kindle covers, gloves, and even lawnmower accessories. Over the years, I've made a few hundred dollars from my Amazon affiliate account so it's worth doing.
You can also create some neat Amazon widgets such as a slideshow of book covers.
You can register at Linkshare to become an affiliate of Barnes & Noble, Kobo, and Apple. http://www.linkshare.com/
Smashwords has an affiliate program also and I have made a little extra money there.
Are your books on All Romance E-Books/OmniLit? They have an affiliate program also.
Do you use any other affiliate links?