The author's interest in middle-grade books is fascinating. I'd love to read a post of recommendations with accompanying rationale, if only to get ideas for books my daughter might like!
How I Live Now, by Meg Rossof, is a stellar novel. There are some really, really good books marketed as YA (Young Adult) (that's the term I know for what he's calling "middle-grade" - maybe that's new branding?), only because their protagonists are teenagers.
"Middle-grade" is the age category before "young adult" https://en.wikipedia.org/wiki/Middle_grade_fiction pragmatically because younger children aren't as interested in the topics that captivate hormone-filled teenagers.
I am ten books into the Wings of Fire series by Tui T Sutherland because my grand daughter likes it. My main reason for liking it is that it gives me a glimpse of the type of anxiety that young kids routinely go through. I think the main reason my granddaughter likes it is because of the dragons.
I really enjoyed Terry Pratchett’s YA books with Dodger and Nation being favourites. Maybe see if you can get access to the librarian at your kid’s school? They’ll be experts