We made this list using arbookfind.com for ATOS reading level and commonsensemedia.com for maturity level.
Books for 5th Grade Reading Level, age 9 or older
Number the Stars by Lois Lowry, Newbery Medal, ATOS 4.5, age 9+
Bud, Not Buddy by Christopher Paul Curtis, Newbery Medal, ATOS 5.0, age 9+
The Phantom Tollbooth by Norton Juster, ATOS 6.7, age 8+
Where the Red Fern Grows by Wilson Rawls, ATOS 4.9, age 9+
A Wrinkle in Time by Madeleine L’Engle, Newbery Medal, ATOS 4.7, age 9+
D’Aulaires’ Book of Greek Myths by Ingri d’Aulaire, ATOS 6.6, age 8+
The Lightning Thief: Percy Jackson and the Olympians, Book 1 by Rick Riordan, ATOS 4.7, age 9+ – better if already familiar with Greek mythology
The Sea of Monsters: Percy Jackson and the Olympians, Book 2 by Rick Riordan, ATOS 4.6, age 9+
A Little Princess by Frances Hodgson Burnett, ATOS 6.0, age 9+
Books better for readers that are also age 10+ maturity level:
Where the Mountain Meets the Moon by Grace Lin, Newbery Award, ATOS 5.5, age 10+
The Westing Game by Ellen Raskin, ATOS 5.3, age 10+
Harry Potter and the Goblet of Fire by J. K. Rowling, ATOS 6.8, age 10+
Harry Potter and the Order of the Phoenix by J. K. Rowling, ATOS 7.2, age 10+
Hatchet by Gary Paulsen, Newbery Honor, ATOS 5.7, age 11+
Harry Potter and the Half-Blood Prince by J. K. Rowling, ATOS 7.2, age 11+
Harry Potter and the Deathly Hallows by J. K. Rowling, ATOS 7.2, age 12+