Stamp Perforation - The Somerset House Years 1848-1880, by R Simpson/P Sargent.

Published by RPSL, 2006.  H/B, 370pp.

A very comprehensive, and the first in-depth, look at the early perforating machines and perforation process of stamps in Great Britain. Includes an illustrated guide to the constant and non-constant perforation varieties of the British and overseas stamps produced at Somerset House, and listings of the stamps that were perforated. Illustrated in black & white and colour.