She's your kid, so the choice is yours, but I think you are an entirely unsalvageable moron if you choose not to confront your child on something like this to find out what exactly is going on. For instance, questions I think you'd probably want to find out the answer to:
Who gave her the books?
Why is she reading the books?
Why is she not telling you she is reading the books / hiding them from you?
What is she doing when she is hanging out at her friend's and who else is there? What influence have they had on her reading the books?
Has she read about any other religions? If so, why not?
Religion is not necessarily a cult, but it is cult-like, especially when you have adults influencing children -- or adults getting their children to influence other children. There is a difference between respecting beliefs and someone trying to convert your child to a religion, which appears to be what is going on. Eighth grade is too young to make a decision like that against your (ie: the parent's) core beliefs without your full knowledge of the circumstances.
Either way, you should talk to your daughter and force some answers out of her. Personally, if it were my daughter, I would get to the bottom of this and expose her to other religions if she actually was interested and help her make a decision, regardless of what that may be. If you are an atheist, she probably doesn't know much about the others, and she should make an informed decision.
Also, I'd probably have some words for whatever dickhead adult was going around pushing religion on my children without my knowledge if that were the case, and possibly result in forcing her to not hang out with her friend if I couldn't trust the people around her / trust that her friend isn't being instructed by her parents to do shit like con her friends.
Frankly, it is NOT normal to start reading religious books at her age just because she knows someone that is religious, especially without you knowing about it, too. Something is amiss. Don't let your daughter be controlled by people other than you in order to satisfy the liberal agenda some of the fuckheads on this forum push. She's your daughter, act like it.