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I know in TNG that all computer interfaces are now touch-screen, and that got me thinking: when was the first reference to touch-screen computer interfaces in Sci-Fi made?

Well I had a nice write up about The Guide in The Hitchhiker's Guide to the Galaxy (due to the amount of blogs and articles which mentioned it) which predated TNG, but I happened to find this link from another scifi stack exchange question: Fictional origins of touch and gesture technology

So I investigated about the "opton" (mentioned by DVK) from Return from the Stars (1961), by Stanislaw Lem and it turns out it uses a touch interface. Here is a quote from the book since that answer didn't mention it:

Chapter 3:

The bookstore resembled, instead, an electronic laboratory. The books were crystals with recorded contents.
They could be read with the aid of an opton, which was similar to a book but had only one page
between the covers. At a touch, successive pages of the text appeared on it. But optons were little
used, the sales-robot told me

And I found an even earlier example in Isaac Asimov's Foundation (1951) with the "calculator pad"

Chapter 4:

"Before you are done with me, young man, you will learn to apply psychohistory to all problems
as a matter of course. –Observe." Seldon removed his calculator pad from the pouch at his
belt. Men said he kept one beneath his pillow for use in moments of wakefulness. Its gray,
glossy finish was slightly worn by use. Seldon's nimble fingers, spotted now with age, played
along the files and rows of buttons that filled its surface. Red symbols glowed out from the
upper tier.

Also mentioned in one of the answers (by DJClayworth) is a non-fiction example, the "memex", described in As We May Think by Vannevar Bush in 1945:

And his trails do not fade. Several years later, his talk with a friend turns to the queer ways in which a people resist innovations, even of vital interest. He has an example, in the fact that the outraged Europeans still failed to adopt the Turkish bow. In fact he has a trail on it. A touch brings up the code book. Tapping a few keys projects the head of the trail. A lever runs through it at will, stopping at interesting items, going off on side excursions. It is an interesting trail, pertinent to the discussion. So he sets a reproducer in action, photographs the whole trail out, and passes it to his friend for insertion in his own memex, there to be linked into the more general trail.

A possible slightly earlier mention - 1941 - is Robert A Heinlein's generation ship story
Orphans of the Sky, consisting of two parts: "Universe" (Astounding Science Fiction, May 1941) and its sequel, "Common Sense" (Astounding Science Fiction, October 1941). The two novellas were first published together in book form in 1963. "Universe" was also published separately in 1951 as a 10¢ Dell paperback. These works contain one of the earliest fictional depictions of a generation ship.

While I'm not sure if it has a modern touch screen, as part of the description of how the ship was built to last for generations, it specifically mentions non-button controls - you put your hand over some lights to activate things. Not sure if that needs an actual touch or not, but it's the same sort of idea. Its been years since I read it, so I can't remember if had the modern idea of touch over a changeable display rather than touch over fixed buttons (Though it may well have had some feedback for slider-style thrust controls, at least). But since one of the character does some interaction with the ships computer later, it seems likely that it had at least the eqivalent of a touch keyboard.

The May 1941 Astounding is available at the Internet Archive.

P. 24, Joe-Jim unlocks the door to the Main Control Room:

He found what he sought, a man-sized door, closed, its presence distinguishable only by a faint crack which marked its outline and a cursive geometrical design on its surface. Joe-Jim studied this and scratched his right-hand head. The two heads whispered to each other, Joe-Jim raised his hand in an awkward gesture.

“No, no!” said Jim. Joe-Jim checked himself. “How’s that?” Joe answered. They whispered together again, Joe nodded, and Joe-Jim again raised his hand.

He traced the design on the door without touching it, moving his forefinger through the air perhaps four inches from the surface of the door. The order of succession in which his finger moved over the lines of the design appeared simple but certainly not obvious.

Finished, he shoved a palm against the adjacent bulkhead, drifted back from the door, and waited.

A moment later there was a soft, almost inaudible insufflation; the door stirred and moved outward perhaps six inches, then stopped.

P. 29, the Ship's design:

The long-forgotten engineer-designers employed by the Jordan Foundation had been instructed to design a ship that would not—could not—wear out, even though the Trip were protracted beyond the expected sixty years. They builded better than they knew. In planning the main drive engines and the auxiliary machinery, largely automatic, which would make the Ship habitable, and in designing the controls necessary to handle all machinery not entirely
automatic the very idea of moving parts had been rejected. The engines and auxiliary equipment worked on a level below mechanical motion, on a level of pure force, as electrical transformers do. Instead of push buttons, levers, cams, and shafts, the controls and the machinery they served were planned in
terms of balance between static fields, bias of electronic flow, circuits
broken or closed by a hand placed over a light.

On this level of action, friction lost its meaning, wear and erosion took no toll. Had all hands been killed in the mutiny, the Ship would still have plunged on through space, still lighted, its air still fresh and moist, its engines ready and waiting.