PROVIDENCE, R.I. — His colleagues taunted him for writing a 600-page book about cancer. "Good luck," they said.Dr. Siddhartha Mukherjee told the story to an audience of about 600 at the Lincoln School in Providence Thursday night. He was introduced by Robin Boss, the sister of the late Alexis Allen Boss, who established the endowment that brings a distinguished speaker to campus every other year.