Quick Q about Warrior Cats

[brachy]

I bought DD a Warrior Cats book today, I had no idea there were so many series and books. DD has book 1 of series 2...does she need to read series 1 first before moving onto this series or are they standalone series?
She is loving the book so I will buy more, is it worth stopping her with this book and starting from the very beginning?

My children are hooked on this series. They tell me that the first series is the best, although they are still clamouring for the new one from series four.

I think I'd be inclined just to buy her book one from series one when she wants the next one, or perhaps get all of series two and then read series one? I have to admit this is one fo the few series they have read that I've not also read, and although I have been frequently told all the plot I can't remember much!