What has the author Raoul Illig written?

There is no author named Raoul Illig. It's possible you're thinking of Heribert Illig, a German author known for his controversial theory of "The Phantom Time Hypothesis".

This hypothesis claims that the period between 614 and 911 AD, a time spanning several centuries, never actually existed. This theory has been widely discredited by historians and archaeologists.

If you're interested in learning more about Heribert Illig and his work, you can find information online, but remember to approach the topic with a critical eye and consider the vast amount of evidence that refutes his claims.