Salem's Lot links to The Wolves of the Calla, but only because there's a character from that story that pops up. Insomnia more links to the final book, The Dark Tower. The Dark Tower series pretty much lies at the centre of all of King's works (which are all subtly tied to each other in some way, anyway). The Stand would link with the appearance of Flagg (and also with a place that Roland and et al visit during the fourth book, Wizard and Glass), as would The Eyes of the Dragon. But really, I don't think there's anything major that you need to find out from other books that would detract from the enjoyment of The Dark Tower series. What you'll probably find is that if you read more King books after The Dark Tower, you'll spot things that might have been linked or made an appearance.
I second Boneman by saying that carry on, mostly because you don't want to get distracted and forget the smaller details of the overall plot, and also because The Wastelands KICKS ASS. It's my favourite of the series and one of my favourite books full stop.