You may be referring to ... from IN A WRITER'S WORKSHOP Twain writes ... It was by accident that I found out a book is pretty sure to go tired, along about the middle, and refuse to go on with its works until its powers and its interest should have been refreshed by a rest and its depleted stock of raw materials reinforced by lapse of time. It was when I reached the middle of Tom Sawyer that I made this invaluable find. ... I could not understand why I was not able to go on with it. The reason was simple - my tank had run dry; it was empty; the stock of materials in it was exhausted; the story could not go on without materials; it could not be wrought out of nothing. ... I took it out one day and read the last chapter that I had written. It was then tat I made the great discovery that when the tank runs dry you've only to leave it alone and it will fill up again in time, while you are asleep - also while you are at work at other things and are quite unaware that this unconscious and profitable cerebration is going on. ppg 198 & 199 MARK TWAIN IN ERUPTION edited by Bernard DeVoto - Capricorn Books 1968