To quote my high school biology teacher (and please understand, this was an all boys school and we all know what biology topics are of particular interest to a teenage boy. Excuse the crassness of the analogy), "Writing should be like a girl's skirt: Long enough to cover everything, short enough to keep it interesting."
Since your book is on a scientific topic, the bulk of the language needs to be clinical and speak to as objective of an analysis as possible (everyone is biased on everything... but it should be checked against when analizing hard data and agreed facts). I think that the "Note from the Author" is fine to speak on your personal opinions on the matter in a way that isn't as dry or as clinical (sometimes people need to hear moral support which can't be quantified by science... other times, people need to hear a joke about the topics discussed (I tend to write a lot on literary topics or political or legal topics, and I find that sometimes mixing clinical terms with more vernacular terms can help get the point across. But this tends to be included in the main text, which is about subjects that are not hard sciences but philosophical issues.).
In terms of a scientific paper, I would say it would be a good spot to point out what your research does not support and why (especially if the data presented doesn't support a specific part of the conclusion one way or another). It may be a good place to speak about the personal reasons this is important to you or about the people in case studies who impressed you through their attitudes and outlooks, despite the data findings or dispense advise to people who are looking for some comfort that the numbers cannot give.
Another idea is to show arguments you disagree with (try to play the devils advocate when describing the argument, as people are more forgiving if you give it a fair shake but logically explain why you disagree). Or some places to look into for further research and opinions.
If you can get someone to, perhaps have another respected person in your field or near it right a forward... or even allow someone to offer a rebuttal.