I tend to agree. The content of a book is only slightly diminished by a handful of such errors. Most of us are passionate about something. When someone writes about that passion and is in error we can't help but notice. I see it the same for authors who get a location wrong or a historical event like, oh, Paul Revere (sorry, couldn't help but add a current reference that nobody will get in a year, lol).
But if we stumble over it while reading and it disturbs our flow, whether language or fact, we owe it to authors we otherwise enjoy to send them a note. Or rant about it in our blogs.
edit - I believe "an historical" is also still correct, if falling-into-disfavor, English?