When you've written as many books as Stephen King, chances that some of your characters and locations bleed into other stories is high. IT is one of King's most well-known novels, and it's influence has touched nearly every corner of the author's vast library of stories. With Derry being one of King's most haunting locations, and the Losers' Club and Pennywise the Clown being such memorable characters, it's no surprise they're brought up in a number of his books.
King created a vast, interwoven universe within his novels. These are some of the most memorable times IT, Derry, the Losers, or Pennywise are connected to another of his works.
Connection: The Dark Tower VII: The Dark Tower (2004)
During a battle that pits Jake Chambers and Father Callahan (of Salems Lot fame) against a bar full of vampires, Callahan uses his new-found faith and a scrimshaw turtle to hold back the hoard so Jake can escape. In doing so he becomes overwhelmed, and he is ultimately torn to shreds. The scrimshaw turtle flies from the father's hand and slides under a nearby table, "and there (like a certain paper boat some of you may remember) passes out of this tale forever."
The paper boat King asks his Constant Readers to remember is the same one Georgie Denbrough chased at the beginning of IT before meeting Pennywise. The wording in both books is nearly identical, too. IT reads: "...all I know it was still afloat and still running on the breast of the flood, when it passed the incorporated town limits of Derry, ME, and there it passes out of this tale forever."
Connection: 11/22/63 (2011)
The hole Jake Epping uses to travel back in time in 11/22/63 only spits him out in 1958. Before dedicating himself to living in the past for five years to stop the the attempt on JFK's life, Jake decides to see if he can change events on a smaller scale by stopping one of his adult student's father from hitting him with a hammer when he was a kid. The child version of his student happens to live in Derry, where IT takes place.
While wandering the city, Jake comes across Bev Marsh and Richie Tozier - two members of the Losers' Club. Jake meets the kids in September of 1958, which is only two months after the kids beat Pennywise the first time. Due to their recent victory, the two aren't concerned by a stranger coming up and asking them questions. The kids mention the child slayings, the clown, and even ask Jake if he knows about The Turtle.
Connection: Misery (1987)
Long before famed author Paul Sheldon was held captive and tormented by hyper-fan Annie Wilkes, he lived in Derry, ME. Not only did he live in the haunted town, he grew up next to one of the Losers. At one point in Misery, it's mentioned Paul's mom used to spend her time with "Mrs. Kaspbrak from across the street." This Mrs. Kaspbrak is the mother of Eddie Kaspbrak, the hypochondriac member of the Losers' Club.
Connection: Dreamcatcher (2001)
Mr. Gray - the main villain of Dreamcatcher - heads up to the old Derry Standpipe at one point in the novel to administer his infection into the town's water supply and finally get his revenge. When he gets to the Standpipe, he finds out it's gone - it collapsed in 1985 when the Losers' Club defeated It as adults. In the pipe's place is a statue of two children and a plaque that reads, "TO THOSE LOST IN THE STORM MAY 31, 1985. AND TO THE CHILDREN, ALL THE CHILDREN. LOVE FROM BILL, BEN, BEV, EDDIE, RICHIE, STAN, MIKE - THE LOSERS' CLUB."
Casting an ominous shadow over the heartfelt tribute from the Losers' Club is the words "PENNYWISE LIVES" that Mr. Gray sees spray-painted in red over the plaque.