Not necessarily. Tons of websites out there are heavy on images, and many of them need those images to be available soon. You don't - we're talking about a textual book with some illustrations, not a picture book. (Even if it was a picture book, people only read one page at a time.)
Meanwhile text isn't particularly strenuous on browsers. 40 pages * 500 words/page * 10 characters/word (spaces, punctuation, some HTML markup) = 200,000 characters. A fraction of a megabyte.
So don't worry about it. Not yet.
40 pages could be a lot for a mobile device, so for that it would be worth considering paginating the content. Perhaps on desktop too, if you want the aesthetic of it.
Unless pages are important concepts you need to account for, and you should try to avoid that if you can (pages are just so arbitrary), then you don't need to break it down beyond chapters.
Worry about AMP later.