Designing a graphic book for an E-reader has a different proportion than for a print book.  One or two panels per page show well.  To check for text readability, reduce the image to real size, about 3.5″ x 5+” / 9 x 13 cm.  (this image original: 132ppi, print size 6″x4″ / 15 x 10 cm / text, 30 pt)