How to increase book weight range for a same-level chapters
I was surprised to hear that the weight range numbers are limited, I expected to get a number input field (so I could enter any number value) but instead, the weight choice widget is a select options type, and for the Book module weights it really is limited to values between -15 and 15, and it still is in Drupal 8!
This is an important accessibility issue, but it doesn't seem so easy to fix once you dig into it. This was on the radar for a long time: Reordering fails with more than 31 book pages in a book that turned to Add a scalable weight select element which circled back to Improve #weight select element behavior based on the number of items and finally in D8 Books: Better Sorting. Maybe you can test that patch and help get it implemented in D8? There is an Accessibility Group you can join and advocate for better weights handling without javascript.
In the meantime, there is a number of modules you can use to reorder book pages a different way.
Book helper seems to be the best to try out first.
There's also a comparison page: Comparison of Node/Entity Ordering Modules