Non-fiction books usually aren't designed to be read cover-to-cover.
It's ok to jump straight to a chapter or section that's relevant to your topic, or to skim material looking for important ideas.
You want to make sure that you're not understanding the material out of context, but you aren't obliged to read the whole thing.
The Table of Contents is often a first stop to get a sense of which sections are going to be most useful. It's normally at the front of the book, before the main text begins.
The Index is incredibly useful; use it to look up names of people/organizations as well as key subject terms or events. The index is an alphabetical list and will provide page numbers of where those terms are found within the book. It's normally at the end of the book, after the main text ends.
Don't discount the Introduction; this is often the best place to get the kind of broad overview or foundational information that can be so hard to find.