Empty space at the bottom of a page is almost always caused by your paragraph style Keep settings. Keep in mind that these do exactly what you ask!
“Keep all lines together”: no matter how long a paragraph is, if it’s too long for a page it will move to the next one. “Keep with next paragraph”: no matter what, this is super-glued to to next paragraph and it will never break between them at the bottom of a page. “Keep with previous”: the same, but for the preceding paragraph.
Deselect all of these options for all of your paragraph styles. For a basic text book, you only need a few!
1. Basic widow/orphan settings for plain text: at the start and end of a paragraph, keep 2 lines together.
2. Basic Heading settings: keep all lines together, and keep the entire heading with the next paragraph.
That’s really all. You still may have room for ‘several’ lines at the bottom of a page, but if you look at the next page it should be obvious why that is. (If it starts with a heading, for example. Typically, you wouldn’t want a heading at the bottom of a page.)