This is on both the iPad and iPhone.
If you open a book (monograph is what I am focusing on here) and set the View to turn off "Scroll View" so you swipe to read pages (just like a Kindle), you cannot go back a page. Scrolling to the left will take you to the Reading Plan page, not the previous book page.
Ideally, I'd like to always have non-bible reading plans switch to "non-scroll view" but realize that isn't really a bug, but a design issue. But when i am in this mode, I should be able to move back and forth between pages if necessary.