JMO, but based on his apparent knowledge of coins, antiques, perhaps poisons, not to mention the people who happen to have the same names as his alleged aliases and other, people with similar, erm, talents - presumably he must be very widely read in quite niche subjects. This was often well prior to the internet. Presumably he must have frequented libraries or bookshops where specific books would need researching and ordering?
How I wish those records, particularly libraries, in the areas he’s lived, were accessible.