there are 3 books that you need to colect to open the portal
first one you find in a safe in an office the code is 3581
you get the second one by interacting with Stans pictures you need to touch them in order from youngest to oldest
and you get the third one by collecting 6 mushrooms located around the map (the hardest one is located next to the big foot picture)

addicional easter eggs
you can shoot the UFO's to make the big UFO appear in the sky
if you shoot the S on "Shack" the S will fall
there is a small % that you will enter backrooms when falling to the hole
there is a talking fish in a shop
there is a talking box underneath the couch

there is one thing that i haven't solved and that is the graves on the back of the map