the book has hashkafa as well as halacha…
“Otherwise, single women would not be allowed to wear hats ever as its a sign of being married”
there is no issur for a girl to look like as is shes married. i once hears an interesting p’shat on the idea why a divorced woman should cover her hair…
before someone gets married they feel like they have to look pretty so they should get attracted to a man. so the rabbonim were matur for a girl to have her hair. (I heard there are sefardim who make the girls cover their hair)but when a woman is divorced she realizes that it isnt her beauty that will bring her a successful marriage. (I know there are rabbonim that let divorced women uncover their hair-it was just a thought on the topic i once heard)