To be fair, I do agree with some parts and some books. If someone says that Mein Kampf is their favorite book or if they highly recommend To Train Up a Child, then that's a red flag. However for the majority of these books I wouldn't think twice about someone owning any of these books or even enjoying them and I find the idea that ownership or enjoyment of these books to be a red flag odd.
One: They enjoy annotating their books. Some people find underlining and highlighting and writing notes in their books to be a satisfying activity. And usually to be able to mark up a book, you have to have your own copy. You can't just borrow a book from the library or a friend and then jot down your thoughts in it. Well, you could. But that's just rude.
Two: It's easier to go back to a book when you own it. You can reference it whenever you want. All you have to do is walk over to your shelves and it's there.
Three: I think it's important to remember that there's been a lot of talk about book bans lately. I'm sure some people are worried they won't have access to a certain book at some point. It might give them peace of mind to have the book just in case. Even if it's a book with questionable material, it can be disconcerting to think that maybe one day some books will be prohibited.
Regarding there being a red flag because someone likes a particular book, I would say it's important to remember why people read. Usually it's for one of two reasons, if not both.
One: For pleasure
Two: For understanding or learning.
So if someone is reading for pleasure, or if reading is how they relax, then they might like a book simply it helps them destress.
Some of the books on these lists were YA and children's novels (a book that frequently popped up was Percy Jackson). Citing reasons like, adults need to outgrow children's books or they want more adult themes in their books, these opinions sound like someone can't grasp the idea that all some individuals want is to read a comfort book. If someone says that Percy Jackson is their favorite book, it might just be because they can unwind while reading it. There is no automatic reason to believe that they don't read any other books or that they've never read anything else or that they like other books (what a crazy thought - you can like more than one book!).
In a similar vein, one genre of book I noticed came up repeatedly was self-help books. The general consensus was that these books are boring and that they're repetitive. Another complaint was why read more than one self-help book? If they work, you should only have to read one. The problem is that just because you find something repetitive or boring, doesn't mean everyone else does. Also, if someone discovers they enjoy a certain genre, it makes sense that they're going to read other books within said genre. Usually someone reads a self-help book to learn or grow their understanding. It's possible that if someone admires a particular self-help book it might simply be because they think that book helped them a lot.
Here's the thing. Just because someone likes a book, it doesn't automatically mean they think that book is the pinnacle of literature or that that book is going to become a classic someday and be remembered for generations. It could just be that they like that book.
Moreover it's important to remember we don't have to read nonfiction to learn or to deepen our understanding of something. The beauty of stories is that they can teach us so much about humanity and human nature, even (or especially) our not-so-pleasant characteristics.
That's part of what makes The Great Gatsby (a book I saw on several lists) so great. It's filled with these vapid, self-indulgent, myopic people. People I would never want to be friends with in real life. Despite this off-putting cast of characters, The Great Gatsby still has this enduring and an almost Ecclesiastes-type thought. You can everything and still feel empty.
Reading helps us understand uncomfortable ideas and realize unpleasant realities about the world.
Truth can be distressing at times.
And sometimes what makes a book so magnificent is deeply flawed characters committing deplorable actions, because that's reality, even if it is distressing. It shines a light on how we really are.
We were created to be seekers of the truth. One way we can do that is through reading. And in that search we might something we treasure even if it is unpleasant.
And I think that's necessary to remember: you can find something valuable even if it highlights the disturbing facets of humankind.
Greatness and the bitter truth are not incompatible.