King technically wrote his own follow up to Salem's Lot,a short story called "One for the Road" that can be read in Night Shift. It's not so much a sequel as it is a glimpse into the state of the town and how the vampires were not as gone as Ben hoped they would be at the end of the novel. It's a pretty quick read and I recommend it.
Hollywood did not pull from that story, or even anything in the original novel at all, when they made A Return to Salem's Lot.
"This is one of those sequels that feels "in name only", like they either just ran off and did their own thing, or had an idea for their own film in mind and agreed to slap the franchise brand on it so as to get funding.(read more on Noel's blog)"
As of this date, this is the last sequel to a Stephen King film left to cover, and Noel has been released from having to watch and review these films. Unless of course one of these new remakes they are planning is successful enough that they decide to start making sequels to those. Or someone decides to make a 10th Children of the Corn film in order to hold on to the name. Let's all hope that doesn't happen.