I haven’t seen the Photoshop Classroom in a Book in a long time, so I don’t remember what methods it suggests. However, in general, yes, it’s better to keep your images as RGB as long as possible, placing them into InDesign as RGB. Then you can convert to CMYK when you make a PDF. Or, even better, if your printer accepts RGB PDFs, then just export as PDF/X4 and let them do the hard work.
The question of out of gamut colors is a huge, hairy one, but often ignores the key issue of “important detail.” For example, in many RGB images, if you check the Gamut Warning in Photoshop you’ll see a lot of problems… but they’re in areas where there is not much important detail. Then, if you soft proof the image (preview how the CMYK conversion will appear) and see that you’re not going to lose very much detail, you might decide not to worry too much about it. For example, a lot of the out of gamut range may be in dark shadows.
If you have important detail that is being lost, then yes, you can adjust the whole image, or just those areas, squeezing and tweaking until that detail is maintained in the conversion. But do that in RGB! That way, you don’t lose all the RGB goodness.
Bruce Fraser and I wrote about this extensively In the book “Real World Photoshop.” You can likely find a used copy online. Almost any version from after 2000 will have this material.
.
