If you purchase a book from Amazon that you didn't intend to, Amazon will refund your order if you request a refund within seven days of the purchase. You can contact Kindle customer service here: Amazon Kindle Customer Service.
You can also call Amazon at 1 (888) 280-3321.
If you'd prefer, you may also visit Amazon's Manage Your Content and Devices page, click the "…" button under "Actions" for the title you'd like to return, and select "Return for Refund." That page is located here: Amazon.com - Manage Your Content and Devices.