One of my personal frustrations with technology textbooks is that they very often contain inaccuracies. Not only technology textbooks, but textbooks in general often contain inaccuracies - especially textbooks that contain what are purported to be historical facts. Authors and publishers are often persuaded to change "facts" to suit political/corporate interests. Whenever conflicting historical stories are encountered, it becomes the responsibility of the student/learner/informed citizen to seek out the stories and situations that may have led to the historical "gray areas."
Some of the historical "gray areas" in MIDI and electronic music textbooks concerns the year(s) in which MIDI was invented, and what keyboard was the first to implement MIDI technology. And so, here are some MIDI history links to hopefully clear up some of the historical confusion.
Prophet 600 vs. DX7