Wraparound book cover: How best to do this?

Question

I've designed a wraparpund cover for a trade paperback using three separate composite images from Photoshop Elements 8: front, cover, back cover, and spline. Each of the three parts consists of layers for background, text, and on the front cover, the image of an airplane.
The backgrounds for all three parts have been made from a single photo of a mountainside partially obscured by clouds. I've managed to crop the images and join them side-by-side to minimize the hard line of demarcation between the spline and front and back covers, but they are still visible. And although the lines will be camouflaged pretty well when the cover is folded around the book, I'd like to make them less apparent.
Each of the three images has been added to the InDesign cover document directly from PSE8 as a PSD file. I'm still a novice at this, so I'm assuming that this means the layers are still separate (?). Is it possible at this point within InDesign to edit the demarcation lines so that they are less noticeable in the printed cover?
Thanks in advance for any suggestions.

Answer 1

I'm almost overwhelmed by the willingness of forum experts to give of their
time to assist strangers muddling around in the wilderness.
On the subject of Photoshop PDF, I certainly didn't understand what I think
I understand now: saving in that format doesn't close the door on any
subsequent changes I want to make in that file. I can back out of it, change
something and resave it as Photoshop PDF without losing any quality. Also as
I think I understand because you told me, going from PSD to PDF and back to
PSD negatively affects image quality.
But that's not the initial concern I had, which is this: When working in
InDesign and ready to check out how my cover would look in PDF because
that's what I need to upload to the print-on-demand service, the window that
opens with options included "High Quality Print" or something to that
effect. The POD service specifies "Print Quality," which I interpreted as
meaning the same thing.
But when trying to save from PSD to PDF, the only option I have is Photoshop
PDF, and the options box that opens up has very few selections. I can't
remember exactly how it looked, but I think it allowed me to select quality
from low to high, but the absence of more available selections led me to
believe that converting to PDF from Photoshop was a lesser quality option
than doing so from InDesign. From what I've been reading today, I'm more
confused than ever. If going direct from PSD to Photoshop PDF to uploading
to the POD site results in a high-quality final product, I'm a happy camper.
Which brings me back to another major question, which is if I can build a
PSD cover in Elements and go direct to a PDF that meets or exceeds
requirements, why did I get so wrapped around the axle about whether to
purchase v5.5 or 5 in order to build the cover? What's the advantage of
building the cover in InDesign with pieces created in Elements? I've no
doubt that InDesign is far more capable, but it appears that I don't need it
to accomplish my relatively modest goals. I've already bought it, so I can't
back out now, but the question lingers.
And then there's the issue of dealing with the more obvious hard edges at
the top where the front and back covers meet the spline, and that original
question still lingers as well: is there something I can do with InDesign to
make these hard transitions less noticeable? I'm not concerned about the
lower portions of the cover where mountain meets mountain, but the
mountain-cloud boundaries are much more obvious.
Due to file size and email limitations, I have attached the first of two
versions of the cover. It's a regular(?) PDF of the InDesign version made
with the front and back covers and spline as created as separate pieces in
Elements. I'll send the second after this one finally departs.
Please don't laugh when you view them. I'm trying hard . . .
Tosh