Of course you may already have a blog or business website and they would certainly be good places to promote your book. But what you really need is a dedicated book website with the name of your book as the domain name. This is not for the paperback edition but for a digital PDF version which purchasers can download immediately upon payment and read on their computer.
The paperback and Kindle version will be made available on Amazon of course and you can mention that fact with the appropriate links on your book website. The PDF version can either be available at a reduced cost or you can charge the same price but offer an additional bonus or two. Depending on the subject matter of your book, bonuses could be companion a book on a related topic, worksheets supporting the content of your book, a piece of software or anything else with a perceived value.
Many people these days prefers the immediacy and convenience of purchasing digital products online. If your website does a good job of peaking their interest often they will make the decision on the spot and want to access the material right away. Sometimes it can lead to purchase of the printed version so be sure to include a link to the book on Amazon.