A directory of Europe's royalty and nobility.
According to Wikipedia:
Following World War I and the fall of many royal houses, noble titles became easy to masquerade due to the lack of a central regulating authority on granting titles; this made inclusion in the incorruptible Almanach de Gotha even more essential. If a noble title was not listed in the almanach, it could be presumed that the title was self-created and therefore invalid. Inclusion of lower nobility was never even attempted, as that was seen as the task of each country's own nobility or corresponding institution.