There are very few stories where the canonicity can be confirmed, I can't even tell which of those in the bottom section Michael and Bryan have even read.
From the Wikia
In late 2010, Samantha Robertson, an editor at Dark Horse Comics at the time, approached comic author Gene Yang, who was best known for his American-Born Chinese series. After "some conversations" with her and the creators of Avatar, Michael Dante DiMartino and Bryan Konietzko, Yang was contracted to write three graphic novels to serve as a direct sequel to the original series.
This is revealed in an interview with the Gene Yang:
AvatarTheLastAirbenderOnline.com – What role are Mike and Bryan playing in the project? Are they co-authors etc..? (How much are they playing a part?)
Gene Yang – Mike and Bryan have been involved from the very beginning. We had a couple of phone conversations where they gave me ideas that eventually made it into the scripts. They’ve read over every outline and every script that I’ve written. They give me notes on how to get the comic to feel more like the show. Working with them is one of the best parts about this project. They and their entire crew are consummate storytellers. I’ve learned a lot by hearing how they think through a story.
I would surmise from that, and the fact that this trilogy directly leads up to the formation of Republic City, this is canon.
All but two of these (Gym Time and New Recruits) are considered canon. This is because they are all publications endorsed by Nickelodeon. New Recruit is listed as a 'bonus story' which is what casts doubts on it's canonicity, as well as the characters being fan created. Gym time features a dead avatar and various enemies, and thus does not fit in canon (it is similar to the bonus material on the DVDs featuring valentines day, and chibi characters.)
This is a novelization of the last four episodes of the series, as such they are literally canon with the show as far as I can tell
Other material of dubious canonicity:
The Earth Kingdom Chronicles
The Last Airbender Movie Novelization
Battle of the North
Trial by Fire
The Avatar's Return
Trivia and games
The Ultimate Pocket Guide
Brainbenders for the Four Nations
Mindbenders and Brainbusters: The Ultimate Avatar Challenge
The Avatar's Return
Graphic novels/comic books
The Last Airbender movie tie-ins
The Lost Adventures
Avatar: The Last Airbender — The Art of the Animated Series
Italics indicate items that definitely do not share cartoon canonicity, ~ items are yet to be released.