Reddit reviews MONTAUK

Well first of all, there's the book series that the whole show was based on; the Montauk Project book series. However, these are more like guide books without a plot and claiming to be non-fiction, but they are interesting. If you're willing to put up with the authors' many, many tangents in regards to aliens, Nazis and "sex magick", there's quite a bit of useful information about how psychics work, what MKUltra did, how isolation tanks function, what the psychic kids were like, how possessions and "spies" worked, what exactly the Upside-Down (or rather "the Outside" as it was originally called) is and how the "actual" 1983 gate-opening event happened.

There's also two novels based off of those books called Montauk and The Montauk Monster. The former is about a group of teenagers vacationing in Montauk who end up having to deal with the psychic kids, government cover-ups and creatures of the Outside. The latter concerns a cop having to deal with more government cover-ups to solve some disappearances and fight more monsters.

I haven't read either of those two books yet, but they both have rather positive reviews so they seem to be worth a read.