MIDI (Musical Instrument Digital Interface) isn’t just for synths any more. It’s not as fast or wonderful as ethernet, or a number of other protocols out there, but it’s still in use. Even if it’s simply tying lighting consoles together, there are plenty of uses for MIDI in the theater.
I learned the basics of MIDI from the book, MIDI for the Technophobe by Paul White. It is fairly basic, and it’s been around a long time, but MIDI hasn’t changed much in the years since it’s been around. It’s a fast read, and worth your time.
And if you are a book type learner, and you are interested in sound, I recommend any of the audio books written by Paul White from Sound on Sound Magazine. His style makes it easy to understand what he’s talking about.
(the link above is to Amazon.com, but it is not an affiliate link. I do not earn any money off the sales of any books through that link. As nice as that would be, Colorado is not eligible for Amazon affiliate programs. There, you have been fully disclosed to.)