Most of them end on either a proof or an excercise just before spewing out a table of definitions or the bibliography.

It'd be nice if they ended with a bunch of practical notes on the subject, papers regarding up to date research on the topic that's graspable enough to understand after reading the book, unsolved problems, maybe even some commentary about the applications of the theorems in neighbouring areas of math or other topics of science like physics and biology. Mathematicians have a very practical mindset when teaching a subject but they often forget that for people to be engaged and excell in a something you must spoon feed them something that peaks their interest.