A book that changed your opinion about something.
There are lots of those around. Practically every non-fiction book I read.
"Testament of Youth" by Vera Brittain changed my perception about women's lives in the early 20th century.
"Not So Quiet..." by Helen Zenna Smith did the same.
"The Big Book of Pain: Torture & Punishment Through History" by Mark Donelly has been quite insightful recently.
"The Rise and Fall of the British Nanny" by Jonathan-Gathorne-Hardy provided some startling insights.