The most important lesson Junior learns in The Absolutely True Diary of a Part-Time Indian is that it's all right to be who he is, a "part-time Indian" split between a largely white world of wealth, privilege, and hope, and a reservation world of poverty, racism, and despair. Junior battles external and internal pressures throughout the novel. His white high-school classmates inflict racist insults on him ("Indians are living proof that niggers fuck buffalo," one tells him) or generally cast him as a loser, while his peers on the reservation see him as a traitor. Internally, he alternates between guilt, shame, and pride as he navigates his disparate experiences. Eventually, he figures out that he is embracing elements of his nomadic heritage. It is OK to embrace tradition, and it also acceptable to embrace the hope represented by the white person's world, and to leave the reservation.
I realized that, sure, I was a Spokane Indian. I belonged to that tribe. But I also belonged to the tribe of American immigrants. And to the tribe of basketball players. And to the tribe of bookworms. And the tribe of cartoonists. And the tribe of chronic masturbators. And the tribe of teenage boys. And the tribe of small-town kids. And the tribe of Pacific Northwesterners. And the tribe of tortilla chips-and-salsa lovers. And the tribe of poverty. And the tribe of funeral-goers. And the tribe of beloved sons. And the tribe of boys who really missed their best friends. It was a huge realization. And that's when I knew that I was going to be okay.
Junior learns many other lessons in the book, but this one—the discovery of who he is, and that there's nothing wrong with being the way he is—comforts his soul, emotionally equipping him to meet practical challenges.