Increase weight range for child pages in book outlines (Book module)
Many people agree it's a big accessibility problem, but it isn't easy to solve and there always seem to be some obstacle. 

I just posted about this, possible duplicate, very relevant for this question because the listed issues mention various methods on how to approach solving this and the obstacles you might encounter: How to increase book weight range for a same-level chapters. 

The simplest way to solve it might be to rely on an outside sorting method, like Weight, and not on overriding the Book module. I don't have enough expertise to evaluate the difficulty level to modify the Book code, it just seems that way reading the lines and between them on those threads.