Now, you can lock the orientation sensor and pretty much never encounter this issue again (we can't ever really imagine needing to read a text-based book with adjustable font sizes in landscape mode, after all), but it seemed worth mentioning given it goes unchanged since the previous version.
That button also lets you buy the book (in the case of a sample), skip to a specific page, bookmark, look up a word in the including Collins dictionary, view the book's table of contents, and adjust the font size -- there are eight font sizes in all.