When you are in the creation part of your book, you first have to click on your text. A blue frame will appear around it and below a toolbox. This toolbox allows you to modify the appearance of your text (font, size, color, alignment, bold, italic, etc.). Click on one of the following buttons depending on the alignment you desire.
If you want to align your text to the left, you just need to click on the left align icon. For a right alignment, you will need to click on the right align icon and if you want to center your text, you just need to click on the center align icon.
TIP: If you want to have text centered on the page, it's important to also center it within the text block before aligning it on the page.