Stephen King and Peter Straub (who found fame with 1979's Ghost Story, his fifth novel, and who is still actively writing and publishing today) first teamed up to create the absolute classic The Talisman in 1984. The story of a boy, Jack Sawyer, who discovers that he can walk between worlds - our world, and a parallel version called The Territories - where most of us have a twin of sorts, was absolutely brilliant.
It focused on Jack's quest to retrieve a crystal (the titular talisman) from the other side that could heal his mother who dying of cancer in our world, and his befriending of a werewolf - Wolf - who accompanies him on his journey. The existence of parallel universes is a shared theme between The Talisman and The Dark Tower, but it is 2001's follow-up to the novel, Black House - centering on a grown up Jack Sawyer returning to the territories - that is more heavily connected to King's fantasy epic. In the sequel, the older Sawyer, now a retired LAPD lieutenant, finds himself investigating a series of child killings similar to those of the infamous Albert Fish.
In the end, it turns out the killer is seeking children to serve as "breakers": powerful psychics the antagonist of The Dark Tower, The Crimson King, is using to destroy the "beams" that support the Dark Tower itself - the nexus of our world, the Gunslinger's world, and all others. You can read Black House as a simple sequel to The Talisman, but it makes much more sense as a tie-in novel to The Dark Tower.