Instructions for Scrolling View in iBooks.
From the normal (landscape view) of the book, tap the upper right of the screen (area of red circle).
The screen below will appear
Tap on the AA (circled in red)
This screen below will appear. Here you can change the brightness of the display (blue circle) by moving the slider, and change to Scrolling View (red circle).
Tap on the A or A (circled in red) to change font size
From Scrolling View tap on the A A (circled in red) to get back to regular view