Simply, the most useful stuff ever written about interviewing others.
Biagi interviews famous interviewers and gets them to talk about how they do interviews. Not only do we hear professional conversationalists like Ted Koppel telling us what he’s really doing when he’s got someone on Nightline, we also pick up the techniques Biagi herself accumulated after finishing interviews with 40 other interviewing experts.
(Strategy for a great book: X-ray the familiar. Document the hidden structure in an overly visible process. Man, is that informative).