leaf, thank you so much for your reply.
To answer your questions, I've done nothing to convert the images from RGB to CMYK. From what I understand, the conversion takes place when the file is saved in PDF/X1-a.
Yes, my monitor is calibrated.
I've spent hours on the CreateSpace forums and, apparently, this is a common problem with color files. I learned a lot about how experienced CreateSpace users work with their photos for a better outcome with CreateSpace printers.
Apparently, CreateSpace is set up to work with sRGB, and does their own conversion, so I'm experimenting with PDF/X-3 as well. Using Adobe Reader for a first viewing, however, doesn't seem to make a difference in how the files look.
I've attached images of what I'm seeing. The actual print copy is closer to the Adobe rendering although the background color is not actually black, it is a dark, muddy green.
I've been reading about device profiles which become embedded in PDFs and am wondering if I didn't accidentally do that (not that I would know how to do it on purpose!). That might explain why, no matter what changes I make, the renderings always come out the same.