Open the brackets, put the verb in the correct form in the sentence: “I was reading a book when I (to find) ... out it was about my dad.”

I was reading a book when I found out it was about my dad.

The past simple found is used for the completed event that happened during the ongoing action in the past continuous.

Перевод: Я читал(а) книгу, когда обнаружил(а), что она была о моём отце.