Maybe Essential DT reads:
Eyes of the Dragon : This one is where Flagg first shows up, and the 2 characters mentioned in BraheSilver’s spoiler box.
The Stand: Has Flagg in it, as well as ties to book 4. Also the Trashcan Man is neato.
Hearts in Atlantis: Flagg is mentioned in this one a few times. The first story and last in the book have DT stuff.
Insomnia: Very heavily DT related. Its got me in it, its got all sorts of wacky Tower stuff in it, but… I thought the book was garbage myself. Still essential reading, but I think it was a weak book.
The Little Sisters of Eluria: This one is story about Roland that takes place before DT 1. Some characters from this story appear in Black House. It was first published in a book called “Legends”, a fantasy story collection by various authors. It was also in King’s latest story collection, Everything’s Eventual.
Black House: Yep, key elements. Its got a character from Hearts in Atlantis, Crimson King, and probably one of the other characters will appear in a future DT book. But to understand everything you have to read The Talisman first, which isn’t that DT related.
Salem’s Lot: You’ll want to have read this one before you read DT 5.
I think the connection in IT to the DT books is so small its not worth reading.
The Regulators and Desperation are both good, but not really Tower related. They both feature the same characters, but are set in different locations. Maybe you can call it set on different levels of the tower. Still it is fun to read one and then the other and catch all the little differences.
I can’t comment on Bag of Bones as I have not read it.
All the above stuff came from my brain, so its subject to being way wrong. I don’t have all the books with me now, so I can’t verify the stuff I wrote. And I haven’t read all of King’s books, so there could be many more DT crossovers.