His books were published in Spanish, Italian, Flemish, Dutch, English, French and German.
As well as his historic and philosophical writings, Brandt generated extensive correspondence with authors such as George Bernard Shaw, Leo Tolstoy, Albert Einstein, Benedict Lust - one of the founders of natural medicine, Ernst Haeckel, Max Nordau, Gabriela Mistral, Raffaele Garofalo, Russell Wallace and Elmer Lee.