This book is written by Erin Hunter, who also happens to be one of my favorite authors. She is the author of the amazing Warriors series that is very popular here in North America and it is almost entirely rare to not ever see someone reading it or for it to not be in a library. This is the the type of book that someone who is interested in fiction and fantasy would want to read - even someone like me was able to read it, even though I don't read that often.
This book is confusing, but that's because its the sixth book in the series, so if you're interested in fantasy fiction I would recommend to start from the beginning of the series which is "Warriors: The Sun Trail." This author also has two other series of books; one is called "Seekers" and the other series, which is as popular and well known as "Seekers" and "Warriors," is called "Surviors."
So to sum up the book without spoiling it (or telling you the main story which would spoil it for you), the book picks up where the previous book left off. The Star Clan has divided and turned into different groups and when the Dark Forest Sprites start to gain strength, there is no Star Clan anymore so the Thunder Cats (which is one of the many clans in the series) has to step in and try to stop the sprites in the Dark Forest before it's too late.