The importance for family historians, says a spokesman for the genealogy website Ancestry.co.uk, is that these phone books fill the gap between the present and the turn of the century.
Digital access is fueling the explosion in interest in family roots, says Cliff Shaw, who founded the genealogy-themed search engine Mocavo.com last year.
Such genealogy sites, which people join in hopes of compiling their family tree, sometimes include Y chromosome data because it is passed from father to son and can be traced back generations.