The 40 minute documentary focuses on Maurice Sendak, author of Where the Wild Things Are. Jonze developed a relationship with the writer 15 years ago and have remained close friends ever since. Like most documentaries, the film came from a simple desire to capture Sendak's thoughts on the adaptation of his book and quickly spiraled into a lengthier project on his life.
Personally, I'm a big fan of the short story Where the Wild Things Are and Spike Jonze, so I can't wait to see this film. There's no official website, so I can't point you to a trailer, I can however point you to a fascinating interview about the documentary over at New York Magazine.