We don't even need to use the written word. Most things that were printed early on tended to purely be pictures instead of text. It would take some thinking on what symbols to use, but you can communicate a lot of information. Political satire and comics is extremely easy to start up. Perhaps far easier than giving out meaningful information about magical threats.
Like... For our premier comic, I'm thinking this:
Slightly off center, a fat man with a crown of antlers sits on a throne, his back towards the center of the picture while he faces a feast. His hands are buldging, filled with rich cuts of meat, succulent gravy, pies and tarts, that he's just shoveling into his mouth. A young, pretty, innocent serving maid is uncomfortably squirming under his other reaching hand. On the right side of the picture, countless people are shouting, crawling, begging the fat man. These are people of all types; farmers and washer women, smiths and burghers, merchants and miscreants. They're begging for help against the shadowed hands that dog their heels. Tentacles reach out from the edge of the page, grasping at people's throats. Fiendish scaled hands reach up through the earth grasp at people ankles. Dead, rotting hands as well as those of pure bone pull at the waist. Over the entire scene, a small dragon circles, hallowed in light. Its fire breath scourges the undead, the Illithids, and the fiend. A few of the people look up, see the dragon and smile.
After all, when the monsters come in the night, would you rather have a Dragon on your side, or a Stag?
EDIT: I want to arrange it in such a way that all smaller problems are automatically handled with a bit of minor supervision, but that Garin has the resources of an entire organization to draw off of when hunting something big.
This was what I meant. I mean, as fantastic a spy as Garin is, he's one person. Having a few subordinates for him would vastly increase the amount of surveillance that we can commit to. For the most part, I see us having three levels to the organization:
On the bottom, we have a huge number of gossip mongers. Their basic role is to serve as an early warning system for people to trip. I honestly don't expect much. They're there to add more dice to the pool in the hopes that we end up rolling some Nat 20s. For the most part, the gossip mongers are only weakly associated with the organization. Most of these people are nameless NPCs; they could be shopkeepers, inn owners, servants, or other gossips. They may not even know they're part of our intelligence service. As far as they know, they're simply trading gossip.
For the mid-level, we have a the lieutenants that organize and pursue all of the information the gossip mongers gather. Their job is to collate it and look for trends. They're there to help look for conspiracies, inconsistencies and other, bigger picture problems. The captains will work to expand and organize the gossip monger network further. They are the ones that keep contact with the gossip mongers in order to move the information up the chain to where people can act on it. Most of the people here probably have skills specifically related to subterfuge. Most of these people are probably Experts.
The last level is problem solvers/specialists. Once the captains identify a problem, the problem solvers deal with it. That could include putting someone under surveillance, arresting them, or quietly disposing of them. Garin explicitly rests at the top of the heap here. Ideally, we'd have a few Rogues (or perhaps Bards later on) under him so that he doesn't have to do literally everything sneaky. There exists a level between 'Not Happening' and 'Fiendish/Illithid/Fey/Pick One' plot. Garin deals with the later, but the other problem solvers can cover things lower than that.
To be honest, I'm drawing a little bit from gang structure in conceptualizing this. Most of the actual labour is performed by 'outsiders' who aren't truly part of the gang. The actual entry level positions are supervisors of the unaffiliated hanger-ons. The cell structure makes it hard to rip holes in the net while also offering a lot of manpower and coverage.