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