This book got delayed for 3-4 years before it came out. Originally, it was marketed as some sort of encyclopedia containing information about animals and even the posting on the publisher's website today mentions including animals from the spirit world. However, none of that turns out to be true. Instead, the narration provided is more of an in-universe narration from Aang, which leaves some problems in that a lot of the text is simply referencing events that have happened in the show itself (and maybe some comics) but not really new information. Put simply, this is not the book that was originally advertised.
There were also some instances of timeline or lore inconsistencies. Here are some examples. In the first episodes of Last Airbender, Katara mentions that she used to go otter-penguin sledding as a kid. In the entry for that animal, it mentions that Aang brought otter-penguin sledding to the Water Tribes but that is directly contradicted by the aforementioned first episodes of Last Airbender. Another inconsistency is Aang somehow knowing the lion turtles from Wan's story in the Legend of Korra. Except it was Korra who was told this story in the series, not Aang.
Which brings me to my next point: although it has The Legend of Korra in the subtitle, there isn't that much text for that series in this book. I was hoping to have something a little more of substance from that series so this wouldn't come off as false advertising. For example, the Wolfbats entry (name of a pro-bending team in the Legend of Korra series) is narrated by Aang so there isn't any references to the pro-bending team in that entry. Although there are snippets of extra text from characters in ATLA and TLOK I wouldn't liked some more snippets from the Legend of Korra characters to really justify TLOK in the subtitle. As it stands, there were only TLOK four characters (Korra, Bolin, Meelo, Ikki) with five total small snippets in the entire book. It would've been nice, for example, to add an Asami snippet as well to the Sand Shark entry since both her and Korra see that animal in that series and it's the only instance in the franchise where we see that animal.
Those two stars I give are exclusively for the artwork. The artists have done an amazing job and they deserve credit it for it.
I would only recommend this book if you are an ultra fan who wants to complete their collection or really want to own the artwork here. Overall I found this a dissatisfying entry after such a long wait.