In case anyone isn't sure what's going on with Mejis, basically think of it like Japan publicly banning nuclear research due to Hiroshima and Nagasaki to the point of executing even foreign scientists that made the mistake of visiting the country, yet the government is secretly stockpiling all documentation on the subject. The bandit leader was probably a former assassin for the Mejis kingdom, and stole one of the books as a blackmail tool when they tried to eliminate him for outliving his usefulness.
@Kuroageha Probably it's like Tensuru, where there are Red, Blue, Yellow, Green, Purple, White, and Black primordial demons.