To be fair, I've been reading an apparently popular series (24 books in and going, widely enough read that multiple random people recognized it when I made a post about it) and, though I've been enjoying it, there's no way reading it by itself would teach you to write well.

Last night, I read the word "ust" used where the word "just" was clearly intended. There are so many missing commas, unclosed quotes, and random (or absent!) line breaks.

Grammar errors and typos aside, there are plenty of continuity or logical errors. For example, a few books back from where I currently am in the series, I'm still not fully sure I understand the events of the book. The author kept describing a situation where, say, "John entered the room," but then describing the actions Paul had undertaken after entering a room, as though the author forgot which character he introduced one or two sentences previously. The series is military sci-fi, so rank plays a major part; as such, I was never quite sure whether the entering character made things better or worse for the protagonist.

(If John is the one who entered the room and is allowed to do whatever they're trying to do, it's good, the brass is on their side. If Paul is the one who entered and is not permitted, just foolhardy, there will be consequences in-universe.)

Not to malign the series; when I can follow, I like it, which is why I haven't named it in this post. Plus, I've recently read a few other popular novels with the same problems, often worse. I think being an avid reader is a great thing, it just doesn't automatically confer a good understanding of language.

(I hope very strongly to have not made any significant errors while writing this comment; that would be embarrassing.)