I am a big fan of having a central place to focus your marketing efforts. I think a blog or website is essential for marketing your book. You need social media but you need a place for social media to point to, and some kind of blog or website is perfect for that. Of the two the blog I think is better because if is easy to update and add new content to. Your site has your bio, book description, sections from your book, links to your Amazon sales page, your media kit and anything else you can think of.
These two videos walk you through how to set up an Author/Book blog and build out your content. I am using Google Blogger because it is free and is part of the Google family. You may use something different like a Word Press Blog or a web site but the theory is the same.