THE book on the theatre of the absurd is called (not surprisingly) THE THEATRE OF THE ABSURD, written by Martin Esslin. This is the book that actually coined the term. Great chapters on Beckett, Ionesco, Genet, Pinter, Albee, and Adamov. Originally published about 40 years ago, but still in print.
"Inside every actor there is a Tiger, a Pig, an Ass, and a Nightingale. You never know which one is going to show up."
-John Michael Higgins in FOR YOUR CONSIDERATION