While IANAL, there’s more to selling a product than just shipping.
For example, if you have a website that caters to European users, it may be subject to EU regulations, for example GDPR.
What’s written inside your book may also matter. For example, your book may contain a picture of a swastika in the US and be perfectly legal. But at the same time, selling that book to a German customer may be a criminal offense under German law.