The Stephen King novel featuring Derry and the creature Pennywise is It.
It is set primarily in Derry, Maine, a fictional community repeatedly used in King's writing. The novel follows a group of children who call themselves the Losers Club and later return as adults to confront the entity that terrorized them in childhood.
Pennywise the Dancing Clown is the creature's most famous appearance, but It can take many forms. The clown persona helps the entity attract and frighten children, while the wider story presents the monster as an ancient being beyond ordinary human categories.
The novel is sometimes confused with Dreamcatcher or Insomnia, which also use Derry or nearby Maine settings. Those books have different plots and supernatural threats. It is the work most directly associated with Pennywise and the Losers Club.