The pronoun who refers to people; which refers to things. See also: "Which," by James Thurber.
Examples:
The man who just left drives a Pacer, which once was called "the car of the future."
Practice:
(a) Nan's book, _____ was published in May, is now a bestseller.
(b) A writer is a lucky person _____ has found a way to talk without being interrupted.
