Hi Grey Ram!
Thank you so much for enjoying the game expansion. The key itself should be in the main core book (Magnagothica Maleghast by Tom Bloom) on the page where the haunts are located right before the factions, but it would be a good move to add a copy of that into my book as well for ease of access and transfer. Within the community ive worked on this with the symbols are pretty much universally known due to the key and haunts existing in the main book, but thank you for bringing to my attention how that may be confusing for new players. As a quick reference I can state here now that:
Spikes = Hazards
X = Wall
^ = Elevated Terrain
/// = Special Square
⭐️ = Treasure Objective
I hope this helps!!