Chapter.html: currentBook?

I see that on line 1, the Back button’s href is:
href="#/books/{{ currentBook }}"

Where “currentBook” is coming from?
In the ChapterController we have $scope.currentBookIndex, but not currentBook.
Is Index a string that can be ignored?

I know this is a late reply, but I was curious so investigated this project. It turns out there is a typo in the original that the dev team have been alerted so it should be changed soon.

currentBook 
# should be
currentBookIndex

Any update on when this will be addressed?