The novelist and short story writer who first said, "write what you know" was Sarah Orne Jewett. She had reumatoid arthritis and part of her therapy was to go on long walks. During these walks, she imagined the stories she would write. It's curious to me that the person who said to write about what you know wasn't a memoirist or a biographer, but a novelist.
I've written many books. All but one sit in the depths of my hard drive waiting to see the light of day. So, are they all about what I "know"? No, they are not. One is about a blind man whose life became so interesting to me, and for whom so much was not know, that I created a life around him, just so I could commit it to my memory. One is a children's chapter book about children who embark on an adventure together, to escape the fate of being split up in foster homes when their mother is gone. I didn't know any of these stories before I began to write them. I know them now. So, I'd say, "Write what you want to know" would be more accurate for me.
I began my memoir, Breaking the Code, without even knowing that is what I was doing. I was writing what my father told me. I was writing questions and queries. I was writing answers too. I knew nothing of what my father had done during the war. But something drove me to WANT to know more. That is what kept me moving forward. After several years, I finally knew what I was writing. If I had sat back and wrote what I knew, my world would have stayed very small, but in allowing myself to wonder and question, I came to know something far more than I could have ever imagined - my father.
What an adventure this has been. What do you want to know?