The Massachusetts Book Award list has great chapter books that include Newbery winners with a range of books from 3rd grade through 8th grade.
If You Like Percy Jackson, try Noah Zarc: Mammoth Trouble by D. Robert Pease. It’s a Time Travel Action Adventure Series (and the Physics of Time Travel). Then, go backwards in time along the same story line but with A Wrinkle In Time’s Sandy and Dennys in Many Waters.
I found it so interesting the wide range of interests and levels of newly minted 6th graders. They are reading everything from adult literature to fantasy adventure to Karl Sagan non-fiction. I guess it goes to show you that the middle school kid is everywhere on the spectrum from adult-like to still child-like.