Twilight seems to be this year's Harry Potter. Ask any middle-school, high school, or even college aged girl and they have either heard of it or read it...some have even committed certain parts to memory. I will admit that I've read the books. I thought they were decent, maybe a little too long at points, but a good way to waste time at the beach over the summer nonetheless.
I think what makes these books so appealing to so many readers is their ability to relate to them in a way that allows them to sort of escape their real lives and live in this fantasy world with Bella and Edward. Now, it may be difficult to understand how someone could relate to falling in love with a vampire, but, when you take away Edward's state from the equation, you're left with a pretty generic fairy tale.
This fairy tale being the girl who is always down on herself, has few friends or is new to a certain place, falling madly in love with the cute, yet unattainable (at least to everyone else) boy. It's something that arguably most girls want. This is not to say that most girls are self-conscious or don't like themselves, but I think the majority of girls can relate to wanting someone to come and sweep them off their feet. This is precisely what happens in Twilight.
Young women (and even some older) read these books as a way to escape their lives and live a fantasy for a little. In fact, it would seem that this is why we interact with any media. Not that we are particularly unhappy with our lives, but we just want to see what it would be like to live differently for a short time. Chalk it up to wanting to see how the other half lives. Authors make their characters' lives seem so interesting and eventful and it is easy to get lost in this fake world when our own world seems so dull in comparison.
Most of us lead pretty normal lives. We deal with things as they come and try our best to avoid drama. However, there is some appeal in the overly emphasized drama of television, movie, and book character's lives that draws us in. It seems as though as long as its their drama and we could look or sound as glamorous doing it, it would be okay to deal with unreal levels of drama--only for a little, though.