Menschliches, Allzumenschliches, 1878. Recommended translations:

• Human, All Too Human: A Book for Free Spirits, trans. Marion Faber, with Stephen Lehmann, with introduction and notes by Marion Faber, University of Nebraska Press, 1984, 1986.
• Human, All Too Human: A Book for Free Spirits, 2nd ed., trans. R. J. Hollingdale, Cambridge University Press, 1996.

Excerpt:

Often enough, and always with great consternation, people have told me that there is something distinctive in all my writings, from The Birth of Tragedy to the most recently publishedPrologue to a Philosophy of the Future. All of them, I have been told, contain snares and nets for careless birds, and an almost constant, unperceived challenge to reverse one’s habitual estimations and esteemed habits. “What’s that? Everything is only–human, all too human?” With such a sigh one comes from my writings, they say, with a kind of wariness and distrust even toward morality, indeed tempted and encouraged in no small way to become the spokesman for the worst things: might they perhaps be only the best slandered? My writings have been called a School for Suspicion, even more for Contempt, fortunately also for Courage and, in fact, for Daring. Truly, I myself do not believe that anyone has ever looked into the world with such deep suspicion, and not only as an occasional devil’s advocate, but every bit as much, to speak theologically, as an enemy and challenger of God. Whoever guesses something of the consequences of any deep suspicion, something of the chills and fears stemming from isolation, to which every man burdened with an unconditional difference of viewpoint is condemned, this person will understand how often I tried to take shelter somewhere, to recover from myself, as if to forget myself entirely for a time (in some sort of reverence, or enmity, or scholarliness, or frivolity, or stupidity); and he will also understand why, when I could not find what I needed, I had to gain it by force artificially, to counterfeit it, or create it poetically.

Online:
Amazon (Recommended Translation)
Nietzsche Source (German, Free Access)
Project Gutenberg (Free Access)

Cambridge University Press (Recommended Translation)