Something that you read about in a book, that doesn't exist in real life - only it does, for you.
For me it's klah, from Anne Mcaffrey's Dragonrider books. Klah in real life is strong, hot rooibos tea.
Also the chocolate cake that Bruce Bogtrotter eats in Matilda. That's my mum's special chocolate cake mmmmmmm. Though Costco chocolate cake comes a very close second.
What's yours?