S.R.
Hi! My son is 9 years-old and is a very advanced reader. He enjoyed the "Artemis Fowl" series. I believe there are 5 or 6 books. They are shorter than the "Harry Potter" series, so are more backpack friendly. Additionally, my son is an avid feline fan, so he enjoyed a series of books called "Warriors" by Erin Hunter. Before you dismiss it as "girly," these cats are truly warriors and engage in battles with rival clans. All of the requisite fighting, death, conflicts that boys love are enmeshed. Much to my chagrin, my son also loves the "Captain Underwear" series. Hey, if it gets them to read, that's the ultimate goal, right? The books are admittedly hilarious.
Hope that helps a little!
Best,
S. :)