The book menu/scrubber bar permanently sits at the bottom of the screen. It is possible to have it hidden, where just the tab and bar appears as below:
To access the full Book menu, tap/click the button.
The maximized scrubber bar/Book menu allows you to navigate the pages of the book using the scrubber, as you could with the minimized view, but also provides page number information, the book title and access to the highlighting mode menu. Additional information on those features can be found HERE.
To hide the bar, tap/click the button.