Did you know that I read more than just fantasy? Sometimes I read other kinds of books and they can irritate the shit out of me too! Like history books! So I say to authors of history and non-fiction as a whole: just because you’re not writing fiction doesn’t excuse you from basic literary competence.
First, decide at the beginning if you’d really rather write historical fiction instead. There is no shame in that! There is shame in indulging in romantic nonsense (I say that in the old sense of the word; it is not a crack on the romance genre) while you’re portraying yourself as an Objective and Serious Historian.
If you’re going to write dual biography, don’t just talk about the one in context of them being so cruel to the other, while you go on and on about how wonderful that ‘other’ was. While we’re at it, someone can be a hapless victim of fortune and a canny political operative, but seldom both at the same time. Choose.
Don’t use the term ‘no doubt’ unless you can provide a fucking citation. If you’re going to engage in speculation, own up to it and stop trying to mask it with those particular words. No one is fooled.
Occasional quotes from a single source are acceptable. But if large chunks of your narrative consist of uncritical recitation of that source’s text? Jesus. I’ll just go pick up the original memoir or whatever. It’ll read better and smack of an undergrad trying desperately to pad their essay to the minimum length because it’s due in an hour, oh my god.
Finally, adverbs, like ranch dressing, should be used sparingly. You should not douse your narrative in them until they’re soggy with the weight of ‘sadly’s, ‘cruelly’s, etc, etc, etc. Trust the reader to discern if a statement or action was made lovingly, angrily, or not. They’re not fucking stupid.
Follow this advice and your informative history book will definitely be better.