This is the 11905th most frequent Polish word.
księżniczkowy
"Related to or characteristic of a princess."
The word 'księżniczkowy' is used here as an adjective describing the theme related to princesses depicted on the book cover.
Książka miała księżniczkowy motyw na okładce.
The book had a princess-like theme on the cover.
In this sentence, 'księżniczkowa' describes the style of the dress, indicating it has qualities often associated with princess attire.
Jej sukienka była księżniczkowa, pełna tiulów i cekinów.
Her dress was princess-like, full of tulle and sequins.
Here, 'księżniczkowym' is used to describe the decorative style of the children's room, emphasizing elements characteristic of what might be considered a princess's aesthetic.