Neither is better, I've seen stores be more successful on Amazon and others more successful on their own site. It depends who your audience is and how you market to them.
It's fairly easy to set up both, so I would do this:
1) List your book on Amazon to take advantage of all the traffic there.
2) Set up your home base on a Shopify site where people can buy there as well.
I've heard something like Amazon giving higher commission rate for listing their exclusively. I'm personally not a fan of exclusivity because it's not the best for the customer, but that's something you'd want to look into before making the decision.
Founder of
Speed Boostr (Shopify optimization experts, theme customization, custom app development).
Creator of
Shopify Analyzer (first performance analysis tool for Shopify sites, free for the Shopify community).
Creator of
Order Automator (auto tag orders and customers + auto fulfill orders + email notifications).
Creator of
Theme Scientist (A/B testing app) and
Tip Jar (add a tip button to your store).
