When you have read to the end of the book why do you want to flip back to the first page. Tap to go to bottom makes more sense.
And pressing the back button is infinitely faster and easier than doing the gesture especially if you want to go back several steps.
I disagree, swiping to go back can be done anywhere on the display compared to reaching to and touching the back button.
Also, when reading a book you can touch the bottom and scroll through to any page you wish.