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Unless the book is a reference book, where precision matters over understanding, and the writing is meant to be referred to not read and learned from, there are almost NO good reasons for a tech book to be written in a formal (i.e. non-conversational) style. Much of the time, it’s an indication that the author is thinking way too much about himself, and how he will be perceived.
… people read a story differently and remember different elements when the author writes in the first person (from the “I/we” point of view) than when the author writes in the third person (he, she, it, or they). (Graesser, Bowers, Olde, and Pomeroy, 1999). Research summarized by Reeves and Nass (1996) shows that, under the right circumstances, people “treat computers like real people.
So one of the theories on why speaking directly to the user is more effective than a more formal lecture tone is that the user’s brain thinks it’s in a conversation, and therefore has to pay more attention to hold up its end! Sure, your brain intellectually knows it isn’t having a face-to-face conversation, but at some level, your brain wakes up when its being talked with as opposed to talked at. And the word “you” can sometimes make all the difference.