Q:

A biography of/about/on

Which do I use, in the sentence 
"I'm reading a biography ___ Winston Churchill" ? 

A:

A biography of Winston Churchill is the story of Churchill's life. 
A biography about Winston Churchill implies to me that it's the story of someone's life, and the subject of the book is Churchill.  But if it were the story of Churchill's life it would have been a biography of Churchill, so I'd scratch my head and wonder how this book is different from that. 
A biography on Winston Churchill sounds like something a child would say, either informally or because they hadn't learned to use of.  I'd know exactly what you meant, but I'd die a little inside.