Keltham doesn't know the rules of magic or the rules of illusions. From his perspective, anything he sees can be fake, anything he hears can be fake, all of his Messages can be intercepted.
The only thing that Keltham can trust even a little is information that would be expensive for the Conspiracy to create or edit. A single published guide on prostitutes isn't that, because it's easy for the Conspiracy to edit a guide like that to say that some prostitutes offer the service of beating masochistic men, without them having to edit the rest of the book. Keltham is thinking about informational things that are tangled up, where changing one aspect of the thing would require lots of other changes throughout the structure of information. Keltham is thinking about being able to ask lots of questions with an unpredictable structure, that the Conspiracy couldn't have traced out in advance.
Snack Service can guide him to something like that, about masochism, since it actually exists for that. If Carissa Sevar can think of where Keltham should look to find it quickly.
Snack Service can't actually tell Keltham exactly where to look inside a library's archive of all the newspapers to find all the debates, though, that's not really in Cayden Cailean's domain.