When reading how to toggle page numbers and mini thumbnails at bottom of page?

It is not a PDF.

Normally, when I read a book, I have this at the bottom:

However, I accidentally touched, swiped, or gestured and now the book displays this:

All I want to do is get back to the prior view.

• • The mini-thumbnail view is way to easy to accidentally bump and jump to somewhere else in the book.
  • The simple page-number view maintains a helpful stack, so if you follow a footnote or link, you can easily return to the prior page you were reading.
  • And if Apple's listening, it's not obvious how I got into this mode, that it was available, or how to return from it -- attempting to find documentation online about the Books applications has been very problematic. I'd be really useful if an extra long hold on control pulled up a help overlay.

Appreciate the help -- and if there's a more correct name than mini-thumbnail, that might explain why I can't find it.

...actually, you may be on to something.

I got one of those book deals where they send you the book in multiple formats. I am suspecting that both formats are loaded in Books, and as such, depending on which is opened determines which interface is brought up.

The new version of Books on iOS 13..