We have a release date, and it's September 4! This was announced on @NetflixFilm a few hours ago.

No trailer yet, but we can probably expect one soonish.

"Inspired by" is worth noting here, suggesting that Charlie's film strays somewhat from Reid's book. Says Chris Evangelista at /film:

I’ve read the Iain Reid book that inspired the film, and it’s quite good, and pretty creepy. But based on Netflix’s description above – “an exploration of regret, longing & the fragility of the human spirit” – it sounds like Kaufman has changed the story up a bit. Because not once while reading the book did I stop and think, “Wow, this is an exploration of regret and the fragility of the human spirit!” (Source)

September 4! And of course Charlie's novel, Antkind, is due for release July 7.