Is interpretation of known historical facts protectable under copyright law?

Copyright law is relatively simple to understand - you simply cannot COPY someone else's work. And yes, your own work is then copyrightable. Your work can effectively be the same as someone else's work - as long as you didn't actually copy it. So, for example, let's say you did research on WW II, analyzed all of the major battles by all of the major players - Germany attacks Poland, Germany takes over and occupies France, Germany then turns its attention to the Soviet Union, whatever - you can write your own analysis as you see it, as you have interpreted it, reach your own conclusions as to why the Allies won the war and why Germany lost the war - as long you do not COPY anything from anyone else. That is basic copyright law.

Copyright protects the expression and creativity, but not facts. Interpretation as you put it is rather nebulous ... you can protect the book per se in that you can control the electronic rights, but if you want to be associated with the interpretation, or be considered the thought-leader on the subject, then that is starting to edge into publicity rights or perhaps even personal brand

Copyright protects the expression of ideas, not the ideas in themselves or historical facts per se. As pertains to the latter, what copyright will protect is prior accounts or interpretation of these historical facts fixed in any tangible form, whether in words, songs or other expressions of art. As long as you do not include these prior works belonging to other people in your own work without the permission of the owner first sought and received, I don't think you should have any problem pertaining to copyright. Just about any material on copyright or copyright law will shed more light on this.
I also feel the need to point out that proper acknowledgement of the source or ownership of the information (where decipherable) is very essential. Depending on the historical fact you are writing on, that knowledge or information may be unique to a particular group of people forming a part of their history, therefore a full, proper acknowledgement will be required in a case such as that.
I hope you (or anyone else) find(s) this helpful!