FU, invisible ghost

I was riding the metro this morning next to a thin, black haired man who was reading a notebook full of handwritten notes. He had curly hair, a dark complexion and wore horn rimmed glasses. He wore black pants and a striped shirt. He was reading the notes quite intensely. I know it is wrong to pry into other peoples business but I could not help glancing over at the notes. It seemed liked it was a novel or a story of some kind. I saw the phrase "FU, invisible ghost!" On another page I saw "Burke Lake" and then a few lines down, "difficult river." It was all hand written in one of those essay books, similar to the one my son has for his spelling assignments. What could the story be about? I remembered being at Burke Lake Park a number of times and had rowed across the lake with my wife and son. There was no difficult river there though. Who was the invisible ghost?