The first volume in Dark Horse Comics's all-ages, in-canon continuation of the animated series has become a best seller.
The BookScan charts measure the sales in bookstores as opposed to Diamond's direct-market-only charts.
Children's books are the fastest growing graphic novel genre in bookstores, compared to the dominance of older readers in the direct market.
Written by Gene Luen Yang and illustrated by the Gurihiru artistic team, The Promise is the first book in the series filling in the gaps between the original run on the television show and the upcoming sequel Legend of Korra.
Avatar: The Last Airbender—The Promise Part 2 will be released on May 30.