Read the introduction and the conclusion; after reading the book I will look up reviews of the book- if its an academic book, find reviews in an academic journal (I tend to head right over to jStor and look for reviews, especially reviews by other historians I know, like, and trust).

I will write a summary of the introduction usually - or first chapter - whichever the author lays out their main argument or approach. And then skim the rest of the chapters noting somewhere the themes that emerge or interesting bits of evidence, maybe how the author constructs their argument/orders their evidence.

As to controversies... sometimes you can get that from book reviews- especially if the reviewer was critical of the book. Otherwise you will need to search; usually in the opening or opening of a chapter who the author sees themselves agreeing with/ arguing against historiographically. Sometimes this is hard to get; especially if you are new to a particular field, topic, or style of history writing.

Pay attention, write digest-style notes every so often recapping what you just read.