> Have a background image for a book cover that is grayscale. It
> resembles, and may have been taken from, an old engraving.
> I imported it from a jpg file.
>
> It would be nice to have it in sepia like an old time photo. Is there
> a way to tint it in Inkscape?

Create two rectangles the same size as the image. Make one sepia
coloured, put it behind the image, and then set the image as the
rectangle's mask. Then make the other rectangle black and put it behind
the first rectangle. I tested this briefly on a silhouette-against-sky
image I had handy, and it seemed to work but I wasn't very happy with
the result: it made the lights far too sepia-ish and the black too
black, but you could probably get better results by tweaking the colours
a bit.

It seems like you're using the wrong tool for the job, though. Using one
of the Gimp colour tools, or one of its scripts, would give you much
more flexibility about adapting to the contrast in the input image and
jiggling the range of colours in the output.