When I tried to explore in The Crypts, I came across a temporary wall that would go down if you took the book in The Crypts.
I was walking up to the wall before I took the book and spiders, skeletons and creepers could attack me. My dog also tried to fight back trough the walls.
I also could attack the mobs back and I did, because I was getting low on health.

A few minutes I had almost the same situation with a normal 1-wide wall, but they couldn't attack me.