Books for people with mild cognitive impairment or early dementia

This is a book request, but I believe it’s narrow enough to be worth it’s own post.

My elderly father is showing signs of mild cognitive impairment, and right now he watches TV all day because he is an extreme introvert, has breathing problems, and doesn’t feel like he can focus long enough to get into any books. Obviously watching TV all day isn’t helping with his MCI.

He used to read as much as he could, and was the type who couldn’t put a book down once he picked it up (to the point of neglecting his work and all of his responsibilities, but that’s another story for another day).

I’d like to bring him some books, but I think he basically needs something that’s on the reading level and fast pacing of a middle grades book. I also think he’d get a little annoyed if I only bring him “kids’ books.” So I would love your suggestions for books aimed at adults with adult characters who behave like adults but written at a middle grades level. I’ve read some romance novels that would fit the bill, but he’s more of a speculative fiction guy.