The book was originally written for A4, so the A5 format is a scaled down version (65% scaling) to fit within the POD margins. I'd say that it's perfectly readable, but perhaps this might enable you to make a judgement yourself (Full size A4 on the left, A5 on the right and ruler with centimeters along the bottom - zoom on a tablet or phone and match up the ruler with your own.. then you'll know exactly how the text will look):
Hope that helps! :)