Complex sentence: The magnum opus of Bucke's career was a book that he researched and wrote over many years, "Cosmic Consciousness", published the year before his death, in 1901 and continuously republished ever since.
Split into simpler sentences: The magnum opus of Bucke's career was a book that he researched and wrote over many years titled. It was published the year before his death in 1901, and been continuously republished ever since.