From 2018 thread:
Directing Improv: Show the Way By Betting Out of the Way by Asaf Ronen
The problem with improv comedy is that there is no training program for new directors or coaches. Most folks rely on remembering what their teachers, coaches or directors did with them when they were starting out. Some folks are lucky enough to shadow or A.D. an experienced director who is not only good at what they do, but can impart how to be a good director as well. A very select few have theatrical directing or educational teacher training and can immediately carry their skillset right over into running rehearsals.
Regardless of your skill level, Asaf Ronen's book is fantastic for directors of all levels. Specifically geared for improvisation in its modern-day incarnation, this is a definite must-own for those who direct and coach improv ensembles (or plan to some day).
Asaf's book covers all the fundamentals and then some in an efficient matter-of-fact manner: the job of the director, coaching vs directing, giving notes and challenges, side coaching, casting ensembles, structuring rehearsals, creating a format (and a timetable of doing so) and tech & music in an improvised show. He also covers the topics of being a director and performer in the troupe, dealing with issues in the ensemble, working with a mixed-skill level ensemble and even working with children!
Throughout the book are quotes from other improv directors relevant to the topic being discussed as well as a "Stories from the Front Line" appendix with quotes that didn't quite fit elsewhere. Some of the quotable notables are Jill Bernard, Armando Diaz, Jeff Griggs, Mark Sutton (among many others).
While there are some exercises included through the book (and in a small appendix), you'll definitely want other warm-up/scenic resources to supplement the practical directing knowledge presented in Directing Improv (such as Peter Gwinn's Group Improvisation) or your favorite online improv repository.
Even though I don't necessarily agree with everything asserted in Asaf's book, it definitely made me re-examine my how I run and structure rehearsals...ultimately making me a better director. This is a definite must-own for directors and coaches.
2020 Update:
Directors/Instructors should also check out: How to Teach Improvised Comedy: Using the Science of Expertise to Develop Masterful Improvisors
From 2018 thread:
Behind the Scenes: Improvising Longform by Mick Napier
Behind the Scenes is the heftier, more detailed follow-up to Improvise -- so if the free-flow riffing conversational style of Mick in the previous book bothered you, be forewarned it's taken up a notch here. Luckily, there is no Thermodynamics of Improv chapter, so that's automatically a plus!
A little over 200 full pages (about about 70 more than Improvise), BtS is a comprehensive breakdown of both the macro and micro of an improv show: Mick devotes chapters to introducing your team, getting the suggestion, and even the back line as well as editing scenes, creating scenic variety and sustaining a character--all through his specific no bullshit, been-there-done-that lens. Not only could I hear the actual cadence and tone of Mick's jaded voice when reading BtS, I often found myself nodding in agreement with what he pointed out...which leads me to my next point:
Whereas Improvise is definitely more accessible to beginners, BtS works best when you have a solid base of performance experience or have seen a lot of improv shows. There are specific show moments or performance moves he references that are easy to understand for veteran performers but might not be easily grokked by beginning improvisors who are still wrapping their heads around basic concepts and execution. I'm not saying greener performers will not get anything out of this book--they'll get plenty--it's just that seasoned performers will probably get and relate to more.
For teachers and directors, there are plenty of exercises scattered throughout the book, which along with Mick's pointed critiques of what doesn't work on-stage, yield a lot of directed and practical lessons you can carry into your rehearsals.
This is definitely a must-read for intermediate/advanced improvisors who want to start critically thinking about the context of the performance, the mechanics and consequences of presentation and caretaking the show. Teachers and directors should also think about getting this to supplement other director-focused books like Asaf Ronen's Directing Improv.
2020 Update:
Directors/Instructors should also check out: How to Teach Improvised Comedy: Using the Science of Expertise to Develop Masterful Improvisors