2 books · Ages 5–9 · Chapter Books, Graphic Novels
First Graphics: Early Science is a 2-book series for ages 5–9 by Robin Koontz. Each book stands alone — start anywhere.
Start anywhere — each book stands alone.
That said, Apples, Apples Everywhere!: Learning about Apple Harvests and Water Goes Round: The Water Cycle are great first picks.
No particular order needed — browse and pick what catches your eye.