Probably some children's book I can't remember. The earliest book I remember reading by myself would be one of this series (in Polish) https://en.wikipedia.org/wiki/Tomek_in_the_Land_of_Kangaroos?wprov=sfla1

This one was also a popular children's book, but I think it was read to me by my mother https://en.wikipedia.org/wiki/Akademia_pana_Kleksa_%28novel%29?wprov=sfla1