I'm not sure if I found a problem, or did one of the steps wrong. I'm
doing the exercise on page 113, setting up the layers to do saturation
masking. The problem I have is that on step 15, I'm supposed to merge
the Saturated Colors and Commit Mode 1 layers together. The problem is
that when I select the Saturated Colors layer, which is just a copy of
the background, there's no menu item to merge down, as there is for other
layers. Not sure how to proceed, since I can't find a way to merge those two layers together. Everything in the layers palette looks OK before that step.

This is a Known Issue - there's something on it in the errata section of Richard's website, as iI recall. My own solution: - from my notes I think I had to simplify the 50 per cent grey fill layer (commit mode 1) first to get it to merge. Or when creating the grey filled layer (step 5 p110) rather using a fill layer, just use new layer, then fill with 50 per cent grey - then it doesn't need to be simplified first