Are you her grandmother? Wow, I wish my grandmom had been someone I could have talked to on that level. I think what you're doing is nice.

I don't think you should send the books anonymously. Just with a nice note that says something like "Thought you might find these informative. I wish I'd had some good information sources like this when I was younger and first getting into relationships." And just keep in mind that she's still young and it'll probably just take her some time, like it does most of us females, to figure out how to get the most out of her sexual experience. The books will probably help!