I'm assuming you mean nonmainstream attitudes, like empowered princesses. I can also see avoiding Disney princesses b/c then it can become an issue of the kid asking for every thing that's plastered with the image of those gals. (DD discovered Tinkerbell, and thanks to the ILs, she has now had the opportunity to learn that the versions of things with characters on them tend to be, in her words, "JUNK!" compared to non character branded items
We avoided princess for as long as possible, but DD has started discovering them a bit. So I don't have too many specific book recs, but...
DD loves to watch the movie version of Once Upon a Mattress. Princess Winifred (ie Fred) is pretty cool. DD is young enough that the adult stuff goes right over her head. And it's WAYYYY more gentle than the Disney princess movies.
We also watched the modern TV version of Cinderella (Rogers and Hammerstein), with Brandy as Cinderella. It was only an okay version of the play, but it was colorful, pretty progressive, and DD still got the girl in a big gown dancing around scene she was hoping for. Plus, it's hard to find a cast that diverse!
We like The Paper Bag Princess, too.
Maybe check out Barefoot Books? They tend to have progressive stuff. Also, if you look on Amazon, I know I have found a whole string of positive princess books there, and then it recommends another, and another... or there are lists people make on there.