Wow. No clue. I certainly read documents of various kinds, including fan-fic and other net stories, on a green phosphor monitor before any mobile device was available to me. I know my wife used to read Peanut Press books on her Palm device (one of my first visits here was to learn about converting those books for use on her Kindle), but I don't recall reading anything longer than a readme file or a web article (remember AvantGo?) on my Palm III or my Handspring Visor.
I guess the first time I read a book on an hand held device would be on a Kindle, but for the life of me I could not tell you what book that was, and it would have been only since my Join Date here.
I must have a pretty bad memory.