This sure is an odd product. You can read a bit about the background here: https://2warpstoneptune.com/2013/11/25/the-story-of-troubador-press-an-interview-with-malcolm-whyte/
If that doesn’t work, try the archived version.
The book is initially a bit confusing. On each double page spread there are 3 sections – a full page illustration, and then a facing page with a top half that has a story that goes along with the illustration, and a bottom half that has rules for a dungeon game you can play by yourself. What is confusing and gradually becomes apparent is that the story and the game have absolutely nothing to do with each other!
The story has the party coming across all the biggest monsters in D&D, including Tiamat. The adventure has none of that.
The adventure is called, prosaically, “Adventure In The Dungeon”. There’s a map in the middle and some very basic rules about how to move and fight the monsters in each room. The connection to the AD&D rules is by the thinnest thread. There’s even a monster that doesn’t appear anywhere else – the Iron Skeletons of Grusyin.
Anyway, the story is fairly silly. A lot of it is taken up with descriptions of the monsters, mainly the colours of all their parts so that you know how to colour between the lines in the, shall we say, serviceable illustrations.
I guess the next attempt to capture the young market would be the D&D cartoon.
Well on Goodreads, someone has put June. In the interview linked above, Malcolm Whyte mentions that shortly after it was published, James Dallas Egbert went missing, which was on August 15, 1979. So June and July are likely. I’ll happily go with June.