Conversation so far:
Does Voldemort appear in Harry Potter and the Half-Blood Prince?
Voldemort does not appear in Harry Potter and the Half-Blood Prince.
Was there any flashbacks?
In a series of flashbacks, using the pensieve as a plot device.

Question: Who was the author of Harry Potter and the Half-Blood Prince?
Self-contained rewrite: Who was the author of Harry Potter and the Half-Blood Prince?