Spolin, Viola:
Improvisation for the Theater and
Theater Games for the Classroom
These are both collections of sequenced theater games that can be taken out of context or modified to suit a specific purpose. The focus is on freeing the imagination through focusing on very specific, directed goals. Some wonderful in-class large group exercises.

Boal, Augusto:
Games for Actors and Non-Actors
These games focus on spatial arrangements and power dynamics and are hypothetical in nature and therefore better suited for middle school or high school students. They are wonderful scene starters and also work well as tools to explore the dynamics of a specific conflict.

Lewis, Robert:
Advice to the Players
These are acting exercises designed to encourage sensory exploration as well as to assist actors in fully exploring the characters that they are playing. Some of the exercises can easily be modified to help teenage and adult playwrights actively explore the characters and situations about which they are writing.

Johnstone, Keith:
Impro
A really great collection of improvisational games for all ages and levels. Very process oriented. Playwriting process.

Kennedy, Adrienne:
People Who Led to My Plays
A really moving chronicle of playwright Adrienne Kennedy's childhood and early adulthood. This book is useful to playwrights of all ages as Kennedy ties her life experiences and their results to her later writing. An invaluable resource, as well as a source of creative, human inspiration.

Erion, Polly:
Drama in the Classroom: Creative Activities for Teachers, Parents & Friends
A really good source of concrete suggestions and lesson plans.  Most of the suggestions are simple, require few materials and are useful with non-traditional learners.

Wright, Michael:
Playwriting in Process: Thinking and Working Theatrically
This book lays out ways for the mature playwright to unlock his or her creative theatricality, helpfully pushing writers to think "outside the box".