The Playbook: Improv Games for Performers Edited by William Hall
A book of 370+ improv games for performers (and teachers). This book started as a few xeroxed pages stapled together in 1988 and has grown to a very popular guide for improv games and long form improv.
The book is available at the concession stand at BATS Improv in San Francisco, the Stanford University Bookstore and at Improv Playbook.
Starting in February 2009 William began posting a new game every Monday on ImprovPlaybook. (The new games are being considered for the next edition of The Playbook.)