The Anecdote to Which I Was Referring

I found the anecdote to which I was referring; before, I was looking in the wrong book. It's in WHAT WAS LITERATURE? by Leslie Fiedler, and takes place after Harriet Beecher Stowe "had grown more than a little dotty with advancing age" (to use Fiedler's phrase). "The scene which continues to haunt me," he writes, "is of [Mark Twain] being awakened very early in the morning by the sound of this great demented lady playing the organ and singing hymns in his parlor, into which she had wandered uninvited and unnoticed."