All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.
Those two persons have committed two unlawful acts:
First: Violating the copyright of the two real authors, a matter which leads to harming them morally and financially.
Second: Cheating, claiming knowledge and falsely pretending that they are successful authors by attributing the book to themselves. The Sharee’ah prohibits all kinds of cheating. In some Hadeeths, the Prophet, sallallaahu ‘alayhi wa sallam, said: “Anyone who cheats us does not belong to us (is not a real follower of Islam).” [Muslim]
“The one who pretends to possess what he has not been given is like the one who wears two garments of falsehood.” [Muslim]
This Hadeeth applies to anyone who ascribes to himself anything that he has not done. It is well-known that the person who does so is the first to be harmed because he will be pleased with people's praise for him for doing something he did not do. Consequently, he will stop the earnest quest for knowledge which may only be acquired through striving and endeavoring day and night. Those who know the truth should advise such plagiarists to fear Allaah. They should also clarify this matter to people because it is the truth that should be revealed to warn people and to deter anyone from attempting such a crime.
Allaah Knows best.