The Author is not simply an author of a book, he is more.
The author is not just the author of his own work but that of others, which carried his ideas as well.
It is through his ideas that laid the foundations and inspirations for other works; the possibilities and rules for the formation of other texts.
So even though the author need not necessarily write these texts, they are naturally attributed to him as the "authorship".
For instance, Karl Marxís book, "the Communist Manifesto" gave inspirations, concepts and procedures for future works of communism.
Hence, through his ideas, Marx becomes the "author" of Marxism and subsequently Maoist and Leninism may also be attributed to him even though these works were written after Marxís death.