Would the repentance of one who bought and sold a stolen address book be accepted, knowing that its price was low and he did not inform his uncle, the owner of the book, for fear of problems?

Islam has preserved the Muslim's blood, property, and honor, and forbidden aggression against them. Therefore, if the questioner took his uncle's money, he must repent to Allah and return the money to him, either directly or indirectly, such as by buying something he needs. If he does not do so, his repentance is incomplete and unacceptable.