Avatar: The Last Airbender - The Lost Adventures
Avatar Wiki; Dark Horse; Wikipedia

This is a collection of previously published comics, which was released in 2011. It includes comics I'd previously read in a couple of Nickelodeon magazines, as well as a Free Comic Book Day comic from earlier in 2011. There are also comics I've never read before, from various sources, and some stories which appear for the first time in this collection. The book is divided into four parts, the main ones being for stories set in each of the first three seasons of Avatar: The Last Airbender, plus a section with a couple of bonus stories. There were a number of different writers and artists involved in making these comics, but the stories are all fairly fun, if you're a fan of the series. Many of the stories are pure comedy, while others include a fair amount of drama (like the series itself). Some stories provide information that, while not essential to understand what's going on at any point in the series, does help fill in some gaps. Other stories don't really serve any purpose other than entertainment, and do nothing to enhance your understanding of events from the show. However, I believe all but the two bonus stories are considered canonical. (One of those stories is an SD short similar to the animated ones on the season 2 DVD. The other is drawn more in the normal style of the comics, but includes some story elements which simply don't fit within the canonical universe- but which I still think would be fun to see followed up on.) Anyway, the end of the book also has some samples of artwork that can be found in the actual "Art of the Animated Series" book. For details on all the stories in this collection, see the wiki link above.