Sometimes the most erotic experence a person can have is the one carried out through words, rather than acts. Letting the paper and the words become her secreate lover may be just another level for her.
Perhaps it's nothing more than fantasy to her? Maybe she enjoys reading those above mentioned stories, but that's all the farther she wishes to take her thoughts? Maybe she's worried what you would "really" think had she opened up?
Just because people read these stories doesn't mean they want to act them out in real life. I have several kinky fantasies, but that doesn't mean I actually want them to happen. Be careful not to pressure your girlfriend or you might piss her off.
I've always found stories more erotic than videos because it lets my imagination take off...and it doesn't necessarily mean that I want to act out what I read about, it's just an indulgence...
I read these types of stories also. I like to fantasize about them but do not necessarily want to live them out. They usually make me extremely horny and my spouse gets to enjoy the outcome. I like the domination stories and would love to experience it but am not ready to let my hubby in on that info yet. Not sure if he can handle the real me. Don't worry about your gf. You too are probably reaping the rewards of her reading these stories. Ask her about it and maybe she will let you into her fantasies. Good luck.