On Page 130, under the section Tilemap Collider 2D, paragraph 3, it says:
You'll notice that all the sprites on your Layers_Objects now have a thin green line surrounding them[...]
From what I can tell at this point we don't have a Layers_Objects Tilemap -- we do have Layer_Ground and Layer_Trees_and_Rocks Tilemaps however.
This is also referenced in a slightly different way on Page 132, paragraph 2, when it says:
[...] We've just added a Collider 2D to every tile sprite on Layer_Objects [...]
This time, it's referenced as Layer_Objects vs Layers_Objects.
For both cases, I assume this is just still the Layer_Trees_and_Rocks Tilemap, but it tripped me up a bit.