Reproduction

1. Have a book with a version
2. Create a new versioned page
3. Set some content and save
4. Switch the page to unversioned (from the Versions menu)
5. Set some content specific for this version
6. Switch back to versioned content (from the Versions menu)
7. Delete this version only (from the Versions menu)
8. Go to the book page, that still exists as versioned but has no versioned content sub-pages

You'll see a warning "There is no content for this version" displayed, but there is content displayed, the one from the step 5 (unversioned content).

Explanation

When unversioned, the content is stored directly in the book page instead of the top page.

Expectations

Probably, no content should be displayed at all.

The solution has to be discussed to choose what to do.