Would those of you who have the time please read the following book blurb and tell me whether you think it was written by someone whose command of English is severely lacking? https://www.famousauthors.org/richard-bach

Looks native speaker to me. There are a few lapses of punctuation and a couple of odd word choices but nothing I wouldn't expect in a native text.

If it is a non-native then it's a quite remarkably accomplished one. There is absolutely nothing in there that comes close to anything I would call "severely lacking".

What I see is total uniformity of sentence structure - Subject, Verb, Object. Subject, Verb, Object. Repeat ad lib. Sometimes a compound object, i.e. a list. It's the kind of writing I've come to expect from a fifth-grader. Maybe an eighth-grader. The kindest description I can give is "unsophisticated."

Of course, the Encyclopedia Britannica was written for fifth grade reading level, at least when I shopped for one for my kids in 1970, so who am I to complain?