There's no doubt that Dark Tower entries published afterward.
IT's been adapted twice already, once as a two-night miniseries and later as a set of two theatrical films. It's telling that both adaptations have devoted fans, as there's something about IT, and its characters that resonate with just about anyone who enjoys horror. The Losers' Club is almost impossible to dislike, and their united struggle for survival against IT is both harrowing and thrilling. Heroes are only as good as their villain though, and wow is IT - usually in the form of Pennywise the Clown - an amazing antagonist.
IT is full of terrific characters, and while King has said he probably won't ever write an IT sequel, it's not surprising that he's seen fit to have some of them pop up in otherwise unrelated works. As always, King's literary world is sprawling and complex.
Pennywise the Clown
While Pennywise the Clown has yet to ever appear in his full glory again, he's managed to cast a long shadow over many of King's books since, sometimes through quick mentions and other times in small cameos. In the short story Gray Matter, which actually was written quite a while before IT, a man's hair turns white after encountering a giant spider in the sewer, and that definitely screams Pennywise.
In The Tommyknockers, a character named Tommy Jacklin visits Derry, and thinks he sees a clown with balloons and silver eyes staring at him from a sewer drain. Later, in 11/22/63, Jake Epping visits Derry after time-traveling back to save JFK, and feels IT's presence inside a fallen chimney at the Kitchener Ironworks, even hearing a voice in his head beckoning him to come inside for a closer look. Thankfully, he's able to resist the lure and continue on with his mission. Clearly, the Losers may have won, but the essence of Pennywise always remains.
Beverly Marsh
The only female member of the Losers' Club, Beverly Marsh is also arguably the heart and soul of the group. She's certainly easy to root for, thanks to the abusive father and rampant bullying at school she's forced to deal with on a daily basis, all while trying to fight off Pennywise as well. She might've eventually ended up with Ben after defeating IT for a second time, but in 11/22/63, she's still a kid living in Derry and trying to move on after what happened.
After Jake Epping gets to Derry, he approaches Beverly and Richie Tozier to try and get information about the Dunning family, preventing a tragedy involving them being a sort of sub-mission for Jake. Beverly and Richie are rehearsing dances for a local talent show, but both sense there's something special about Jake, in a manner similar to themselves, having all encountered the supernatural and otherworldly.
Richie Tozier
As mentioned above, Jake Epping encounters both IT Chapter Two concerning Richie's sexuality also makes one wonder if he might've been struggling with that at the time too.
Before leaving, Jake teaches Beverly and Richie the proper way to do their dances for the talent show, in what's honestly both kind of heartwarming and an odd diversion from the main plot. Still, King is clearly enjoying seeing the Losers again. Sadly, the time Jake, Bev, and Richie spent together was eventually erased, after Jake undid his changes to the past. Considering the havoc they wrought on the timeline though, he made the right choice, or at least the only one he could.
Mike Hanlon
In all versions of the IT story, Mike Hanlon stays behind for the long haul in Derry after the other Losers grow up and go on to professional success. He's needed to be the lighthouse keeper for their eventual return, as staying in Derry enables Mike to retain all his memories of IT, and help him recognize the signs of IT's impending comeback. Therefore, it makes perfect sense that Mike briefly pops up in 1994 novel Insomnia.
Insomnia is set in Derry, and serves to further tie the town to the Dark Tower mythology, and King's overall multiverse. Multiple mentions of and allusions to the events of IT and characters from that book are made in Insomnia, but it's Mike that protagonist Ralph Roberts briefly encounters in person. Mike is still working as the head of the Derry Public Library at the time, and steers Ralph to books on the titular condition.
Dick Hallorann
Dick Hallorann, who worked for many years as the head chef at the Overlook Hotel, is the only IT character to appear in other King stories that didn't actually originate in IT. He first appeared in King's 1977 book Doctor Sleep, although he eventually ed away, not that that stopped him from talking to Dan again.
Before either of those stories on the timeline though is Dick's appearance in IT, back when he was a cook in the army. and regular at the Black Spot nightclub in Derry. When a racist group burns down the club with patrons inside, Dick is able to use his shine abilities to save lives, including that of Will Hanlon, Mike Hanlon's father. Mike had yet to be born at that point, meaning Dick indirectly ensured the Losers' Club would eventually form.
