What happens when a king doesn't get what he wants?
This is not a trick question. What you should do next depends on what your prep about this front says about King Cyndemund and why he wanted the books in the first place.
Because he wanted the books, right? This wasn't just some misadventure with no intentionality on anyone's part? If it was one of those, Dungeon World isn't going to be able to help you. You can keep going forward with fronts after PCs derail them, explicitly because they're being driven by actors with motivations of their own, whose plans can be derailed without them being annihilated and who will continue to act:
Every danger has a crucial motivation that drives it called its impulse. The impulse exists to help you understand that danger. What pushes it to fulfill its impending doom? Impulses can help you translate the danger into action.
-- "Creating Dangers", from the repo
That goes both for the overarching impulse that drives Cyndemund on the campaign scale and the individual motivations that lead to his plans as expressed on the adventure scale. For the sake of example I'm going assume you modeled him based on ambitious organizations and detail some more portents for you to dish on your PCs. Maybe you'll find what you're looking for here, maybe you won't.
King Cyndemund didn't want these books in particular, he's just all about that ancient forbidden knowledge. Okay, great! Your world is lousy with ancient forbidden knowledge. So, as a first step, Cyndemund could buy out someone important - since the king can't trust his own agents he's probably going to start funding expeditions for the stuff, on the up-high or perhaps the down-low, but either way a band of mercenary adventures like the PCs is going to get clued in on it eventually. And if they don't bite, Cyndemund then claims a resource - someone else makes the big score down the line, Cyndemund gives them a king's ransom, and that's the talk of the taverns for a while. After this point the arcane/planar/cursed force you've decided he found will start making some moves as Cyndemund oopsie-doodles the apocalypse. It's maybe not the apocalypse you planned for initially, but why should there only be one old forgotten way to destroy the world?
King Cyndemund wanted these books in particular, for Important King Reasons. Okay, great! So, pop quiz: is a king with the mandate of Heaven or whatever going to believe that all his plans have come to ruin because of a misadventure with some confused old man? Or is he going to see enemy action where none apparently exists? So Cyndemund influences a powerful institution - now the talk of the taverns is that the entire town the PCs just left got interdicted by the King's Quisition. And when they're out in the wilderness they can see the king's court mages scrying from the sky, searching for answers - this is what observe a potential foe in great detail looks like when you're a paranoid king. The PCs are the answers, and eventually they're going to get found, unless they can do something to hide themselves. Really, this mostly just stands up a new front and dumpsters the old one, since the doom is going to be quite different and much more personal. So congratulations: your PCs averted the apocalypse! I'm sure that'll be a great comfort on the chopping block.
No, you've got it backwards. The books wanted King Cyndemund. Oh, this is one of those scenarios. So tell me: are the malevolent missives of a buried god really going to give up the ghost in the face of one ordinary torch? Or is this new arcane enemy going to be offering up some portents of its own? It could cast a spell across time and space as a more overt sign - the talk of the taverns is that Cyndemund was going to come down on some old peasant like the fist of a merciless despot, but when they were going to confront the old man with the evidence of his crimes, there the books were, like nothing ever happened to them, and glowing with a holy light! Actually it was kind of a mix of purple and a color that doesn't exist and when I close my eyes I can still see-- A HOLY LIGHT. For extra fun, perhaps it's already recruited a follower or toady - the old man has also heard the whispers of the Buried One and he's "a prophet" now. Has he left behind the mortal concerns of petty revenge in the face of this higher calling, or are the PCs about to be done by as they did?
Problems and Intent
Now, these last two cases might be a bit trickier of a sell to the PCs. You risk coming off as the bad GM who punishes the PCs for messing with their plans. Which is why it's important to have your prep in front of you -- the PCs did mess with the plans of a king and/or a god, after all, so you need to present them with the retaliation of a king and/or god in the ways it makes sense to happen, which the PCs can still see coming and take steps to fight or avoid.
And in the first case, from your perspective as a GM, the PCs didn't actually accomplish anything. But from their perspective, they did everything they wanted! They destroyed the evidence, they shifted the blame, and potential bonus: someone's willing to pay them to fix the problems they caused! Now, their actions aren't entirely without consequences; for instance, I'm sure you can imagine some fun dramatically ironic times when they end up working alongside the former king's agents, still obsessing over that fateful night and their fall from grace. And you're probably going to adapt the campaign in response to Cyndemund's more obvious desires for arcane secrets, bring in some new dangers that might not have gotten involved before. But the PCs didn't intend to stop King Cyndemund from seeking arcane power, and so they don't: he continues to do so in ways that are obvious to them. He continues to give off grim portents because he's still a danger.
What usually motivates the PCs to jump into the teeth of a campaign or adventure front is their interest in acting against the impending doom. When the apocalypse starts happening in scenario 1, the doom's going to get way more obvious and the PCs are going to have to get involved with it somehow. If you want to make them aware that they might want to get involved before then, you'd need someone who does intend to stop King Cyndemund from seeking arcane power. Actually, someone like that would be an asset in all three scenarios, with some strike-that-reverse-it as appropriate. Let's call them Demevend. In the latter two scenarios, Demevend can be an ally to the PCs and help them escape retribution. In the first one? Demevend might approach the PCs to hire them on as double agents, especially if they're not engaging with Cyndemund's expeditions already. Or, if the PCs went after that good good royal purse, Demevend might act against them, and confess their suspicions in a dramatic confrontation.