The Paper Princess by Elisa Kleven (all her books are charming and her collage artwork is wonderful)
Ladybug Girl is very sweet (we have only read the first one, though)
Fancy Nancy is fun and teaches good vocabulary since she likes to use "fancy" words
These were all loved by my princess-loving girly girl. If you want more literal "princess" books, let me know, I can probably think of some!