[JamesP81]

So, let's talk chapters. Beginning them, ending them, what makes chapters good, what can destroy a chapter, and the numerous other points I did not list...

The basic rule is that a chapter should detail some part of the story that advances the plot.

I agree with your point about 90 pages being too long. It's annoying and I need a stopping place at some point. However, I think having chapters too short is just as bad (David Gerrold is notorious for this).

I think the one thing that can kill a chapter more thoroughly than anything else is ending it in the wrong place. Being a little short or a little long is forgiveable; ending in the wrong spot is not.

[Oberon]

To my mind, each chapter should be like a mini-story all on its own. In a lesser way, it should reflect classic story structure just as the whole book does: It should have an establishment of conflict, rising action to a crisis, and a resolution.

The conflict, crisis, and resolution should all relate to some relatively small dramatic tension that advances your story. Of course, this can be difficult to do in non-fiction, but it's not impossible. It's a function of storytelling rather than plot.