I didn't like how the first go-round went and forgot some important ones, so here we go again.
*Yes, I know some princesses don't fit these requirements, but they are often said not to deserve to be in the line-up, and for reasons included in the poll. You can take that into account or ignore it when you chose.*
I think a character from a book could fit the line-up very well, and it doesn't affect whether the princess can be marketed as well as, say, whether the princess sings or has a love interest.

I would love to have someone for a book to join.

Jasmine isn't the protagonist of her movie, and it can be argue that Aurora isn't the protagonist of her movie either.

Jasmine's not the protagonist, and I think she has plenty of reasons to stay in the lineup.

As I said before, Jasmine isn't the protagonist of her movie, but no one seems to mind.

Same as before.

same reason

I just do not see how this is a factor. A lot of the books that Disney adapted are popular. Also, since beig a princess clearly does not matter (Mulan), I do not see why it has to be a legend or fairy-tales. People bed to remember that now Merida is included, a lot of the rules do not matter (including this one). The movies based on fairytales are a minority anyways.

Or being based on a legend/fairytale. This requirement has always felt very elitist to me though, I don't like it.