Starting in 1979, various technical manuals printed on a Model 43 Teleprinter.
Staring in 1981, various technical manuals on various computer screens.
My first "real" ebook was A Princess of Mars, by Edgar Rice Burroughs in 1993. It was on a CD-ROM of PD works. I read it on an Amiga 2000, using a text editor.
My first ebook on an ereader was A Trace of Memory, by Keith Laumer -- included in Baen's edition The Legions of Space. I read it in 2010 when I got my JetBook Lite. At the time I believe it was in their free library, and I jumped at the chance to re-read one of my favorites from high school.