Book came out six years ago, I think we're out of spoiler territory at this point.
And rosebud is a boys snow sled.
Fuinny anecdoate about that actually. As I understand it when Welles made Citizen Kane he picked the name rosebud for the sled becuse the film is of course modeled after William Randolph Heasrt's life and is to some degree a paradoy of his life and a snub.
Apparently Welles had inside information that Hearst named his girlfirends pussy, "rosebud," and it was a further snub.