Not familiar with the Stranger Things universe?
I've seen shirts, hats, keychains, etc.
If fiction is more your speed, then you'll want to wait for the spring release: A still-untitled novel by Gwenda Bond - author of the Cirque American, Supernormal Sleuthing Service, and Lois Lane novels - that will explore the backstory of Eleven's (Millie Bobby Brown) mother as she underwent the MKUltra experimental program.
The series is currently in production on it’s third season which will most likely premiere in 2019
Netflix and Penguin Random House are teaming up to expand the Stranger Things universe with a series of books, which will soon include a prequel all about Eleven's mom and her involvement in the totally real Central Intelligence Agency experiment Project MKUltra. Both titles are set to be released this fall. Project MKUltra, which was an actual Central Intelligence Agency program, is how Eleven got her mental powers while being experimented on under the watchful eye of the nefarious Dr. Brenner (Matthew Modine).
The novels will be a great way to add in extra dialogue about the show, especially those parts which were briefly mentioned, but probably won't be looked at again.
Fans of Stranger Things are going to be very happy to hear that there are a whole heap of books on the horizon, one of which is a prequel novel.
The Stranger Things books will initially be released under the USA and British Commonwealth publishing teams of Del Rey Books, Random House Children's Books, Cornerstone Publishing, and Penguin Random House Children's UK.