The value is that the author is credible as a military historian and he has made a conclusion based on historical inquiry which is the object of the exercise. His Age of Fighting Sail gives him credibility as one of the best books on the subject available.

So, based on his record, his conclusions can be considered valid until shown or proven otherwise.

If modern authors conclusions are not valid, then are you saying that the only thing that has value historically is primary source material? I have found credible secondary sources to be very useful and, interestingly, the evaluation of secondary material is taught in graduate studies in history to be used in conjunction with primary sources.

Sincerely,
K