Both siblings name the books that I would most likely to recommend. Also, Steve Call's "The Deal of the Century" and "Telephone: The First Hundred Years," John Brooks. The latter is a 1976 book which was commissioned by AT&T as a corporate history. As a result it is both outdated and paints a rather rosy picture[1], but I think that's part of what makes that book rather interesting - it's sort of AT&T's best version of itself at the peak of its dominance.
[1] for what it's worth, John Brooks engages right in the introduction with the conflict of a history funded by its subjects, and the book was researched and written independently
[1] for what it's worth, John Brooks engages right in the introduction with the conflict of a history funded by its subjects, and the book was researched and written independently