I... am not all that impressed. This collection of stories is definitely cute, don't get me wrong, but it adds very little. There were a couple stand-outs - 'Relics' and 'Combustion Man on a Train' (despite the latter's ridiculous title) added nicely to Aang's character in little ways. Mostly, though, these read like brief vignettes and don't contribute that much to the overall story of the show - which is good, I suppose, since it prevents the show from feeling like there were plot holes. A few of them bridged seasons 2 and 3 nicely - they didn't really contribute anything that hadn't already been covered in flashbacks, but it was fun to read/see them anyhow.
My one major complaint is that the art varied widely - sometimes it looked like screencaps from episodes that never aired, and sometimes (as in 'Reach for the Toph') it looked... amateur. I'm all for stylistic variation, but some of this came off as sloppy.
Overall, though, this gets three stars more for not being extraordinary than for any particular failings. It was a lot of fun to get to see the Earth King (one of my favorite characters) again, and I also quite liked the story of how Zuko and Mai became a couple. I've always quite enjoyed them as a pair, but felt like they didn't get very much time together in the show for viewers to see what made that relationship tick; while this story doesn't add a whole lot, it helps a bit, and it's pretty cute. And of course there's a little more Iroh - "Old people represent!" made me giggle.
Oh, one more thing... the 'New Recruits' story at the end was a little bit wince-inducing. I like the idea of including fan-created characters in a brief interlude, but some of them were, well... simplistically silly. I'm not sure if that's just the characters who appealed to the comic artists or if that's most of what they got when they called for submission, but either way it didn't work super well. Maybe something obviously out of character like the mini comics at the end of a volume of manga would have suited better?
The art at the back, however, was absolutely gorgeous. I want some of those pieces as desktop backgrounds! And I'll definitely keep an eye out for the official art book.
Full disclosure: I recieved a galley copy of this book from the publisher via Netgalley. No external considerations went into this review.