lark100,
sepia brown is NOT a single Pantone spot color.
A shadow in such an old image is almost black.
Therefore one needs a conversion like this:
Use the original brightness B, but add a little
tint by choosing a moderate saturation S and a
yellow-ish hue H.
Convert RGB to Grayscale, convert back to RGB.
Change color by HSB. For instance:
B as before, but S=0.2 and H=20°, or so.
Duotone, as suggested by C.N, is a good idea, if
the book should be printed by K and ONE Spot.
But which Spot should be used ? There are many
candidates on p.3 here:
<
http://www.fho-emden.de/~hoffmann/swatch16032005.pdf>
If the book contains ordinary color images as well,
then the modified grays have to be simulated by
CMYK (convert modified RGB image into CMYK for
the actual process).
Best regards –Gernot Hoffmann