For years, I've been going to playwriting workshops, devouring scripts, seeing as many shows as I could afford, writing like a maniac, and trying to understand the strange mix of people, time, space and stuff that is a play. If you don't train in theatre, you don't have that frame of reference - concepts of space, symbols, negotiation, status, exposition, time, subtext, audience engagement, disruption, gesture, ritual etc. These areas are rarely explored in writing workshops, yet they are essential for an understanding of the medium.
Fiction and film writers are well served with thoughtful overviews, but I searched high and low for a playwright's equivalent of John Mullan's 'How Novels Work' and Alexander McKendrick's 'On Film-Making'. Edgar's 'How Plays Work' was a great start, and Steve Waters is even better. I particularly love that he gives context and examples not just from plays, but also from film, poetry, fiction and music, so we see universal concepts at work. And - oh my! - he also gets specific about language, e.g. the effect of past tense, which most other playwriting books overlook.
This isn't a 'how-to' - it's a highly readable discussion of the structural bones underpinning drama, aimed at the drama and literature student, playwright and lover of theatre. Rewarding, inspiring and generous.