In a letter to his publisher, Nietzsche informed him that he had a new book ready, Part One of Thus Spake Zarathustra;

"Today I have some news for you: I have just taken a decisive step - I mean,one profitable to you. It concerns a little work, scarcely 100 pages long, entitled:'Thus Spake Zarathustra', a book for all and none.It is a poem or IT IS A FIFTH GOSPEL, or something which has no name; by far the most serious, and also the most happy of my productions and one that is open to all".
[Nietzsche, letter to Schmeitzner 14/2/1882]