The thing you need to make sure is that this is a world where books and printing presses exist. Otherwise, there probably won't be enough copies of anything to justify the expense of scribing the books. (Though if he is that popular, there might be). There is the economics to think of.
You also need to make sure your character's are literate and might actually spend some time reading these things. If the books are scribed, they might be too precious to just let litter around. So you need to make sure that there is a "path" that will lead the players to the adventurous folk books... either they can read it themselves, or they need to be of the type to tell their adventures at a bar, where everyone will then smirk because that was the story in Adventurous folk last season. Go to Comment
This reminds me of something I saw on TV, I can't recall if it was the Twilight Zone, or Amazing Stories, but the vignette was called 'Typewriter of the Gods' and it was alot like this, but the writer discovered it's power to make what he wrote true, and got in all sorts of trouble by trying to bring people back from the dead. Don't remember much more than that, it has been a few years since I'd seen it.
Otherwise, well written, but I don't know how well I would be able to work it into a game.
A good plot useful in a variety of backgrounds and even genre.
The Evil in the Church could be the liche like soul of the original mage (who is buried in the church or perhaps was cursed by the town cleric), the insane priest who came back as a zombie (who is now some crazed evil demigod), a gateway to Ravenloft or similar world (undead world even), or all the Evil energy that the undead townsfolk are not using (it is "pooling" so to speak and is now intelligent after the centuries). What ever it is, it should not be something that players can just throw damage at it and it will go away. No, this thing needs to be taken down through puzzle solving or roleplaying. That fits in with the entire scenario
The Evil should be tied to the townfolk. As they begin to destroy it, the townfolk should be effected adversely. As they die of course, they should be resurrected as normal evil zombies. Go to Comment
You may wish to read a little closer, as a number of your comments were answered above.
"A great necromancer(perhaps a Cancer Mage, if you like the Book of Vile Darkness) created an unholy disease. It infects the body of a corpse, ripping the soul back to the body and reanimating it in some undead form, anything from a zombie or skeleton to a wight or shadow. The disease causes no ill effect(directly), it simply places the person's soul back into their dead body, giving them back their total mental faculties, morals, ethics, beliefs, etc... without bringing their body back from the dead."
That is why the undead are alive, it has nothing to do with the evil in the church. Your other main point, "the PC's going in, guns blazing" it exactly the point of the plot. Its supposed to teach them to look and listen instead of shoot first and ask questions later. Go to Comment