The King James Bible for Gameboy is one of the stranger early etexts that I know about. It was an unlicensed cartridge produced by the Christian game company Wisdom Tree.
The highlights are that the Gameboy Bible had an unusual cartridge, the ability to search for particular words in the bible, and two biblical minigames. I imagine the cartridge would have been most useful for school children having theological arguments during playtime. Robert Busa may well have been impressed by this collision of technology and religion.
In 2009 there were a few official ebook collections published for the Nintendo DS, based on franchises like the Far-Away Tree and Artemis Fowl, as well as public domain classics. You can turn the pages of these e-texts with the DS stylus. I wonder what psychological testing would make of that?
The lesson from these cartridge-based ebooks is that almost any device with a screen will eventually display a narrative text. They also suggest that the dedicated ereader was not inevitable, and point to the possibility of Nintendo or a similar game company releasing some sort of ereader/handheld console. Such a device would almost be redundant in a world of smartphones and tablets, but I could see Nintendo giving it a go and almost pulling it off.
The variety of the ways in which e-texts can be read – on tablets, gaming devices old and new, mp3-players, computers, as well as devices designed for that purpose – makes me suspect that dedicated ereaders like the Kindle are going to end up as a historical curio, like Hula Hoops. The Project Gutenberg people are right to digitize all their texts into the universal ASCII format. All that needs to happen for contemporary e-readers to become redundant is the invention of a tablet that can switch from LCD display to e-ink mode.