I love the book series Warriors, I started them last year (5th grade) and found them a very interesting series. Erin Hunter started writing the Warriors series somewhere between 2002 and 2003. I think the series is a good reading level for 5th and 6th graders. The book series follows a "kitty pet"(House Cat) named Rusty, who dreamed about the forest, one day he traveled a little too far and was ambushed by Thunder clan (one of the 4 clans in the forest) cats and fights back. After he was pinned down, Rusty was invited to join Thunder clan and live in the forest among the clan. He is then named Firepaw and he trains with his soon to be the best friend Graypaw. He gets made fun of for being a "kitty pet" and that's the beginning of his journey throughout the Warriors series.
2nd series Warriors New Prophecy
3rd series Warriors Power of Three
I suggest this series for young readers.
P.S I didn't get the full cover of any of the books.