Hi Jamis,
I think there’s an issue with a test on chapter 6. I own the ebook, version P1.0 Feb. 2019.
This test doesn’t pass for me:
But removing the rotation_z transformation the test will pass.
I suppose that a previous errata updated the transformation matrix m but not the resulting normal vector n.
This is my normal with the rotation_z:
x: -0.4149850363522584, y: 0.8620719608670172, z: -0.29089405956569586
Hope it helps!