At the Millbrook Literary Festival last month, I listened to Bryan Bunch explaining how he did the research for Before Eureka: The Adventures of Young Archimedes, his YA historical novel about the famous mathematician, scientist, and inventor.
Since Archimedes was known throughout the Mediterranean world in his day (the third century B.C.E.), there is a decent amount of information about his public life. Not so much about his personal life. The one source Bunch wished he had, a biography of Archimedes by his friend Heracleides, had been lost in ancient times.
So Bunch wrote Heracleides' letter himself.
To me, this letter was almost as impressive a piece of writing as the novel. At first reading, I was so taken in by the authentic style, tone, and content that I thought it actually was written in the 3rd century B.C.E. to Eratosthenes, chief librarian at the Library of Alexandria.
And Bryan Bunch must have taken himself in, too. Once he had finished fabricating this "source," he said, he was able to forge ahead with the novel itself.