The American Historical Association provides a Statement on Standards of Professional Conduct. It is worth reading some parts of the code. The thrust is that professional historians have an ethical responsibility to provide trustworthy, scholarly contributions to the ongoing dialog about the past. They must leave a record of their work, distinguishing between primary and secondary sources.
The Society of Professional Journalists also has a Code of Ethics. Its primary injunction is to "seek the truth and report it". (Note that the historians are more modest about the possibility of always reporting the truth, although they do believe that "that within certain limits we can indeed know and make sense of past worlds and former times".)
There are also Ethics for Writing Non Fiction. Those ethics requires of the writer of such works that he/she maintain "the highest standard that will ensure the content of your work is true and accurate." "Due diligence" is required of the author.
In the book club we have read works by professional historians, by journalists, and by authors of non-fiction books of history who are neither journalists nor professional historians.
It seems to me that we should consider whether any book provides a trustworthy account of the material it presents -- that the author has seriously sought to discover and report factual material, that when the ideas of others are described, they are described accurately, and when the authors own inferences and conclusions are presented they are identified as such.
We should also demand quality in the writing. Is the book well organized. Is its thesis clear and intelligible. Is the prose clear? Have the author and publisher provided the requisite maps, pictures, charts, timelines, and other materials to help the reader.
If we identify clear ethical misconduct, such as plagiarism or undisclosed financial interests, we should be very cautious in accepting the thesis of the author.