The DTI was featured on-screen primarily in DS9's "Trials & Tribble-ations". Agents Dulmer & Lucsly were the only DTI folks ever shown on screen to my knowledge.
DTI, Dulmer & Lucsly, were the focus of my favorite TrekLit story, "Gods, Fate, & Fractals"
by William Leisner
. This short story is in Strange New Worlds II
If you love stories about time travel &/or alternate realities, like I do, "Gods, Fate, & Fractals"
is a must-read.