Custom post field data not saving

Hi,

I’ve created a new plugin and edited it based on this tutorial:
https://wp.smashingmagazine.com/2012/07/24/wordpress-bookshelf-plugin/

The edits are minor, I’ve removed the prices and all links except one for Amazon.

But, when I add the Amazon link to the post editor it saves it and displays it on the page but when I go back into the post editor the link has disappeared.

How can I get it to show?

Thanks
Sarah