My first, gut-level reaction to this was that I'm not sure I care; it's what on the page, no matter who writes it, that pulls me in. I read a lot of Orson Scott Card, for example, even though he turns me off as a person. I realize that tie-ins and seris--and heck, all sorts of things--might be ghost-written. That said, I reserve the right to think about this for the afternoon and decide I've changed my mind. ;) And I just might, because I have these high-falutin' ideas about books being somehow apart from gross over-commercialization, and being art, even though I recognize that sales drive the ability to have art.
Now that I've given myself a moral headache, I also have to admit that I don't read a lot of the YA stuff that looks like chick lit. I'm curious if you find that teens like the books you've references as much as other, single-author types of books. Did they hit their mark, so to speak? Or is the books' popularity just hype?