By that definition of "radical": painful and difficult to read, but yielding enormous positive transformation, then for me Aldous Huxley's "Heaven and Hell" (which contains "Doors of Perceptions"), Erich Fromm's "To Have or to Be", Lewis Mumford's "Technics and Civilization" and his "The Myth of the Machine". YMMV, but for me all of these were "radical" in challenging my purely rational, instrumental, and I think very limited ideas of knowledge, technology and "progress" that I held as a younger scientist/engineer.