<<My "Tree Selection" layer doesn't really show a tree in thumbnail. It looks more like the outline of the tree or a mask with the tree missing. Does this make sense?>>
Somewhere along the line my bet is that there was an invert selection you either forgot or passed over in the text. That your result doesn't look like what's shown in the book is a hint at what happened...but i can't really be sure what you are doing. I've run through the steps again to be sure they are working, and it goes when I follow...but then again, i can't be sure I'm reading the same way you are as I know it pretty well

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Take a look back and see if there is an invert you missed -- or perhaps you inverted when you shouldn't have? This would have happened, I think, on the previous steps...23-36 on 169-171.
let me know!