Magic
Protective Wards
The bindings that keep dangerous books on the restricted shelf — and that can fail when something stronger enters the room.
Wards are protective workings laid over objects and places. The grimoires on the restricted shelf behind the shop counter are held by wards of their own.
Wards are not absolute. When magic of sufficient force moves through a room, they flare briefly and then fail.
In the books
This is established in The Awakening, book 1 of The Arcane Odyssey.
Explore The Awakening