Since some are being a bit blunt here, it is indeed a book. (-_-) But, more specifically, it's the "Avatar: The Last Airbender: Legacy Book."
It's 64-page book which is essentially a collection/scrapbook that Aang passes down to Tenzin. It takes you on an 'interactive' journey through his [Aang's] most adventurous moments, with posters, clippings, cards, and maps he gathered along the way.
References:
https://www.amazon.com/Avatar-Airbender-Legacy-Insight-Legends/dp/1608874478