It's not a true story. 'Where the Wild Things Are' is a wonderful tale made up to spark the imagination of readers and viewers. There's no factual basis to it.
I'm not sure if you can find it for free legally. You might try some public domain book websites, but be cautious as many free sources could be unauthorized.
The full story begins with Max's misbehavior and his journey to the land of wild things. There, he has fun ruling over them but eventually realizes he misses home. It's a tale of imagination, adventure, and the importance of family.