Thus Spake Zarathustra was Friedrich Nietzsche's favorite of the books he wrote, and has been his most popular amongst general readers. Yet some scholars dislike it because of its unphilosophical nature: it eschews jargon and the scaffolding of arguments, which engage only the intellect, in favor of an artistic approach that engages the whole mind.
After ten years of solitude in a cave high in the mountains, Zarathustra wishes to share with humanity the wisdom he has accumulated during this time. He reaches the nearest town and addresses the crowd on the marketplace. He tells them of the Overman: the next step in human evolution, a being who creates their own values, freed from the weight of tradition and morality, and who takes responsibility for their own successes and failures. But the crowd doesn't understand him; his discourse is met only with rude ignorance. Zarathustra then decides to gather a small group of disciples and share his wisdom with them.
The bulk of the book is Zarathustra's speeches on topics such as morality, society, individualism, religion, and how suffering and its overcoming are what give meaning to our existence. While already wiser than most, Zarathustra still learns from those he talks to, re-evaluating his thoughts as he deals with disappointment (such as when his disciples prove to be mere followers), and confronting his own doubts. His greatest challenge, though, comes when he faces the existential test of the eternal recurrence of the same: the thought that our lives could repeat indefinitely without the minutest of change.
The inspiration for Zarathustra came to Nietzsche during one of the long hikes he often indulged in despite his failing health. It was a decade of solitude: his physical condition had worsened to the point of forcing him to retire from his position at the University of Basel, and each change of season prompted him to relocate to kinder climes in Switzerland, France, or Italy. The book took two years to write. Each of its four parts was written in a ten-day period of creative effervescence followed by months of gloom, plagued by terrible, debilitating migraines.
Zarathustra was initially received with indifference at best and frustration at worst. It's a work of philosophy as much as aesthetics: the language is modeled after the Luther Bible and contains numerous references to Homer, Heraclitus, Plato, Goethe, Emerson, and Wagner, to name a few. Later Nietzsche attempted to address the book's lack of popularity by framing the same concepts in a more traditional, approachable manner in his following book, Beyond Good and Evil, but that book also struggled to find an audience.