I found that it didn't like colons in the title, and like you said, `**Abstract**...` needed to be a heading level 2. I also found that the `## Abstract ...` needs to be broken up, not just made into a heading (saw the `\n\n` in your event published). Otherwise, the article event fails to create (that was the last key to making it work).

Some checks might be needed to provide the user feedback on what might have caused a book to fail to publish.