The warriors series is a popular book series written by Erin Hunter. (Which is actually a pseudonym for 4 different authors) The majority of the series revolves around four groups (clans) of cats living in the forest, especially Thunderclan, from which the author draws most of the main characters for the series. These clans must hunt and fight for their survival, and they face many different problems, but you'll have to read it to fully understand it. I would recommend starting with Into the Wild and then reading through the series. You can check out my reading guide for a full list of some of the best orders to read the series in.