'Salem's Lot is King's Dracula. It's his vampire story, and unlike many young horror writers (keep in mind this was King's second published novel), he got it right on the first try. The book was an instant classic, updating the vampire tale for modern times without losing any sense of morality or just going for sex and gore, as later vampire novels by other authors would so often do.
For years, King contemplated a sequel to the book - then years became decades, and unlike The Shining and The Talisman, which eventually did get sequels, a follow-up to 'Salem's Lot just never seemed to happen. That wasn't the end of it, however. Father Callahan, who in 'Salem's Lot is broken of his faith and forced to feed from the vampire Barlow, returns and becomes a major character in several of the final Dark Tower books, first appearing in the fifth novel, Wolves of the Calla.
Here, he is in New York, homeless, but begins to work at a shelter, and soon finds he can sense vampires around him - and eventually begins hunting them. This attracts the attention of The Crimson King, his can-toi (low men), and they attempt to eliminate him, but instead he throws himself from a window, only to wake up (after death) in the world of The Dark Tower, where he eventually becomes part of the Gunslinger's group. Callahan goes on to become such a big part of The Dark Tower that it's almost as if the later books in the series (and especially book five) become a sequel of sorts to the novel.
Callahan's quest for redemption, after all, would have made the perfect plot for a book, and you get that, in parts, in book five of the Dark Tower and beyond.
There are other novels connected to the world of The Dark Tower of course, from Rose Madder to Under the Dome to 11/22/63. Feel free to discuss all these and others below!