"Holy sh-t! I made a movie!" it begins (via Antiquiet). "I started this project a little over a year ago... just me, my drinking buddy, and a crazy idea that we should tell."
The letter says how making the film became one of the most incredible experiences of his life. He got to hear from Neil Young, Stevie Nicks, Trent Reznor and many others. "All I had to do was listen," he said. "I am the luckiest man on earth."
It goes on to philosophise about the magic of humans working together:
"What is it that happens when 4 people turn on, plug in, and really play that makes the hair on the back of your neck stand up? What is it about those moments when you hear something and it immediately puts its hooks in you, and you feel... understood?
"Get together, start a band, sound like sh-t, and change the world. GO!"
Meanwhile, Grohl confirmed he won't be touring with Queen Of The Stone Age as fans hoped after he performed the drums on their forthcoming new album, according to Blabbermouth.
He last appeared as their drummer on 2002's "Songs For The Deaf", and plays alongside QOTSA frontman Josh Homme in Them Crooked Vultures.
Watch the trailer for the "Sound City" movie by Dave Grohl here: