[neverending]

Shaw. That's all I have to say. Well, no it isn't, but Shaw is essential. He's the modern day Shakespeare. His scripts are so multi-textured and the language is wonderful. You'll find plenty of useful monologues there.

Also essential is Brecht. Brecht is responsible for theatre the way it is today. Read everything you can get by him. A man's a Man, Mother Courage, Caucasian Chalk Circle are good starting points.

Since you seem to enjoy the Absurdists, I'd recommend Ionesco. The Bald Soprano, The Chairs (one of my favorite pieces EVER) and Rhinocerous are essential.

And you must read Waiting for Godot. More has been written about this play than perhaps any other piece of literature. It is the quintessential modern play.