The block accepts the spell when cast through a book, but not through the turret. Even if the turret faces other faces of the block, it still doesn't cast the signal.

Version 2.2.2

Redstone Signal is a bit hacky an unintuitive at the moment. Since redstone works entirely based on blocks being placed, Redstone Signal creates a fake block of air that gives off a redstone signal. Because of this, casting the turret directly into the block will not place it.

Instead, destroy that cobblestone 1 below the turret, and it should work.