From wikipedia.org:

Originally published under the pseudonym "Tex Burns". Louis L'Amour was commissioned to write four Hopalong Cassidy books in the spring and summer of 1950 by Doubleday's Double D Western imprint. They were the first novels he ever had published and he denied writing them until the day he died, refusing to sign any of them that fans would occasionally bring to his autograph sessions. The reason given to his young son for doing this was, "I wrote some books. I just did it for the money, and my name didn't go on them. So now, when people ask me if they were mine, I say 'no.'" When his son asked if this would be a lie, he said, "I just wrote them for hire. They weren't my books."