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.