Before starting a book, Robert Caro tries to state its purpose in one sentence or, at most, one paragraph.

“You gotta get it down to one sentence. You say, "Well, you can't get it down to one sentence." Then you say, "Yeah, but Bob, you learned you better get it down to one sentence, or three years from now you're gonna be writing and you're gonna be saying, 'What am I trying to do?'"”

“So before I start a book, I try to get it down to, I said a sentence or at most a paragraph and say, "Really, what is this book about?"”