The index is your best friend in this. The quality of an index may vary drastically but in a good work of history it will include not just names and locations, but hopefully also thematic topics and ideally subdivided by location or era. So a work on 20th century European history might have an index entry for "Communism" but also a sub-entry for "Russian communism" "French communism" and so on. The individual page given by the index might not be enough to fully explore the context of the explicit mention of that topic recorded by the index but oftentimes the surrounding pages or chapters will.

I'd also point you to the fantastic and much lengthier explanation that was given in the askhistorians "Finding and Understanding Sources" series, and most especially to /u/sunagainstgold's answer on how "How to read an academic book" but the other answers and indeed the entire series are all excellent and worth reading as well.