IGN, that infinite resource for geeky males, offers up Ten Books for Your Girlfriend and it’s about what you’d expect (no, really, you think women would like Strangers in Paradise? The Sandman?). Uninspired as it may be, it’s not a bad list, either. But your girlfriend may like more than this (she may actually like Sin City) and may not like everything on it (she may not like Y: The Last Man).
And maybe your girlfriend is the one who introduces you to comics.