When creating your photo book, you'll see that there are empty text boxes below each photo on the pages of the book. To fill these with your own text, simply click on the text box and type in your text. If you don't want to add text to your creation, you can leave these empty as the text boxes won't be printed.
In the bottom of the page, you can also click on icons to change the text alignment, font, size and colour. When you're done, click "Apply" to view your text in the book creation.