I have read the previous four books just for the hell of it. The first three aren't that long. The last two are pretty big, but you have to take into account the fact that the pages are somewhat larger print with double spaced lines. It's not THAT long.
On the other hand I think after reading one, maybe two it becomes pretty tiresome. Not a lot of new stuff offered from book to book in my opinion. I got pretty bored with it. I'm sure it's different for kids.