Wow, thanks for all the answers. I will try to address everything.
Depression: Possibly. But if so, she’s been depressed at least half her life without seeming to be. I will pursue this avenue further.
School: She’s not popular, but she has a few friends. The one that lives in our neighborhood came over a couple of times which seemed to go well, but now they both seem content just to text. She tends to get attached to a teacher, in whatever school she’s gone to. Her grades are only fair, but she does care about them.
Physically, she has frequent headaches (common in our family).
We talk and I think we have a good relationship. When I talk to her about this, she says people just don’t like her, or she just doesn’t like them. Sometimes she seems very rigid in her opinions (“people who don’t like X are stupid!” or “She’s okay, but I don’t talk to her…she’s a prep.” Or “I don’t know why, but I just don’t want any job working with food.”) This black and white sort of view could just be a normal teenage thing, but she seems very sure of her likes and dislikes whether she’s actually tried them or not.
Reading tastes: For a long time, it was only Harry Potter obsession. Since then, she’s begun to read a little more, but it’s not her passion. She reads my books or sometimes takes my recommendations, but doesn’t branch out for herself.
Household situation: She has a twelve year old brother (Mr. Popularity), a stepdad (they aren’t crazy about each other), and a stepbrother, also seventeen (they ignore each other completely).
I myself was a sheltered, introverted kid. My mom and grandma keep saying, “Oh, she’s just like you!” But by this age, I was skipping school, having sex, and smoking cigarettes like any normal kid. 
I didn’t know how to drive because I was afraid and my mom didn’t think it was important. It was a handicap and it was unnecessarily hard to overcome. That’s why I’m pushing my daughter…not just because she might be unhappy, but because fitting in and keeping up is only going to get harder the longer it’s delayed. I’m trying to help her, not picking on her.
I’m grateful for everyone’s point of view.