The book was published at a time when Laschet served as the minister for generations, family, women and integration in North Rhine-Westphalia. According to Der Spiegel, Twitter users drew parallels between a part of the politician's book and the work of political scientist Karsten Weizenegger.
"I would like to apologize, because ... respect for copyright is a matter of respect for other authors," Laschet said, as quoted by the DPA news agency.
The candidate from the ruling Christian Democratic Union party added that the book would be further reviewed to determine whether "there are other mistakes."
Earlier this month, Laschet's rival, Annalena Baerbock of the Green party, admitted that she, too, had failed to list sources used in a new book.