This book is good to read once you've read every novel in the main Warriors series up to Power of Three: The Sight. If you read it before that point, beware: you'll read a whole lot of spoilers.
However, if you've read Firestar's Quest (the first Super
Edition), you'll notice some major errors towards the beginning. It doesn't say anything about the fifth Clan, SkyClan.
Also, if you've read Code of the Clans, there are some major errors on the "Significant Leaders" parts. It says, under the description of every Clan's first leader, that they "Worked with Thunder, Wind, Shadow, and River [always excluding the one it's talking about:] to develop the warrior code", while the truth is that the warrior code developed under the leadership of other leaders over time. The first leaders didn't come up with the warrior code all at once, and the most recent addition was added when Bluestar was a warrior of ThunderClan.
Another major error is under the StarClan section. "Snowfur Speaks: A Sad, Cold Death" keeps on calling Mosskit a "he", when - if you've read Bluestar's Prophecy - Mosskit's actually a she. It also makes it seem like Bluefur actually had her kits in the forest, instead of having them in camp before bringing them out to the forest.
Overall, though, this was a good book that I completely enjoyed reading.