Books can be printed in any multiple of four, so 40 pages isn't uncommon. Make sure you look at recent books because they tend to be shorter than older books. Authors usually get about 28 pages for the story. The other pages are taken up by front matter, title, copyright, etc. Of course, these do have illustrations often. I wonder if paper backs are shorter because they don't have the pages glued to the covers. I'm sorry I can't be of more help. I'm not an illustrator and don't know all of the terms you're using.