So what the title says, this is useful especially for non-fiction books.
so you can see which chapters of the book you did complete so far, y'know? so you have a sense of accomplishment and on this era of so much information you can't just afford to read a whole book, this is true for STEM books with some that are more than 1000 pages, or 300 which is still a lot lol if you have to work trhough problems. This feature could also be useful for Encyclopedias (ok, this one could be more complicated and another whole different approach, just throwing the idea around here). Also the papers made by some conferences are collected as a book but you just want to read some specific chapter for research, or an essay in some humanities book, you get it.
Like in textbooks for universities you only need to read / practice with certain chapters and not the whole book, but you still want those chapters to be recorded on your "read books" that might or might not be continuous; example i want 2, 4,5, 11 chapters to be recorded, how to do this?
i think if the chapters where separated with commas a bit like tagging in some platforms could be useful, like you write the numbers of the chapters on a blank space: like the tagging of bookmarks on Firefox.
The name of the chapter could be improved on later (like crowd-sourcing for interested readers or other more automatic option), but i think if you could at least see the numbers would be such a good feature that could cover the audiences said above and more non continuous literature / non-fiction friendly c: