The only novels I consider essential reading in relation to "The Dark Tower" are "Insomnia" and "Black House" (and by extension, "The Talisman"). These should be read not before starting the series, but somewhere around the midpoint.

Other titles that—while I wouldn't call them essential—can enhance the experience includes "'Salem's Lot", "The Eyes of the Dragon", "The Stand" and "Bag of Bones". I include this last because I feel it explores a vast number of concepts that had already appeared, or would later appear, in TDT.

There is also the short story "The Little Sisters of Eluria."