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He didn't invent anything; it's his own world of "The Lord of the Rings"... A brilliant phrase. Firstly, this world cannot be his own because Tolkien invented it. Secondly, yes, it's possible that he really didn't invent much because everything was invented by Tolkien, and Perumov only introduced a few creatures that he wanted to see there.
Perumov wrote a free expansion! And he added what he thought was missing in Tolkien! It's a matter of taste; those who like Tolkien should read Tolkien, and those who like Perumov should read him!