The only thing that cat be done inside ID is to assign the image a different profile. You can try that, and check the output preview to see if the conversion to the correct color space fixes things. Otherwise, you need Photoshop (and I'd use an adjustment layer).
If it's just an image, open it in Photoshop and save as TIF or PSD and
let us know how that goes.
I'm no color management expert but I don't know how well that would work
on an EPS image.
Nor, actually, do I. You'd need to be fairly lucky in your guessing in any case to find the correct profile to assign.
The book cover's background and photo image both need to be given a more sepia cast. For someone used to a simple sepia filter, my lack of understanding/command of PS, and a callibrated monitor, and the discrepencies of the printing process, are making this a bit of challenge.
I'm able to resave and open the background eps image in anotehr format (tif, psd, jpg) fine. Not sure how to reliably add a sublte sepia hugh from there.
The proof came back with great photo quality, just needs the slight sepia. The background as well. If one attempts to make colour choices based off the proof, does switching from eps to another format cause the colour information to look different once printed, even if nothing has been changed, just the format (no hue adjusting, etc)?
The format of the image should not affect the color values.
I've just done a little experiment, and maybe there is a way to "fake" what you need to do in ID. Make a shape that covers the area you want to shift and fill it with a sepia color, then set the object blending mode for the shape to Hue in the Effects panel. You can vary the intensity by adjusting the transparency slider. This is going to affect everything below the new shape, though, so may be more encompassing than you wanted. I also had some luck with an overall shift by giving the image frame a color fill, then setting the image itself to multiply (use the direct select tool to select the image inside the frame and change the mode in the Effects panel), but I found it much harder to control the color this way.
My choice would still be to use Photoshop where you have real control and can isolate the shifting to specific color ranges if necessary. Lack of a calibrated monitor is a hindrance, though.