His nine symphonies rule today’s orchestral repertoire, yet their composer is still a mass of contradictions.

Leonard Bernstein once said of Gustav Mahler that he was a double man in every single part of his life: “In the work of every composer, we see shifts between one idiom and its opposite, but nowhere are these shifts as extreme and violent as with Mahler.”

As a Jewish musician at the height of his fame and powers, torn between his dual role as composer and conductor, Bernstein was uniquely placed to offer insights on the great Austrian symphonist’s psychology. But for many of the rest of us, Mahler remains an enigma. When examining Mahler’s work, it can be tough to see through the dizzying complexities of his personal and professional life and give a clear appraisal of his legacy. Firsthand accounts of his temperament and demeanour are often confusingly contradictory. For some, he was a God, for others, a tyrant. He was loved and loathed in equal measure, and often for the same reasons. So who wasGustav Mahler? And can we get to the bottom of this double man?

By the 1890s, Mahler had become the most celebrated conductor in the...