Nietzsche, who had been a student and a professor of philology, had a thorough knowledge of the Greek philosophers. Among modern philosophers, his reading included Kant, Mill and Schopenhauer, who became major targets of criticism in his philosophy. He also mentions reading Hegel at the age of twenty. Late in life he read Spinoza, whom he called his "precursor", in particular for his criticisms of free will, teleology and his thoughts on the role of affects, joy and sadness. Nietzsche, however, opposed Spinoza's theory of conatus, for which he substituted the "will to power" (Wille zur Macht); and he replaced Spinoza's formula "Deus sive Natura" (God or Nature) by "Chaos sive Natura". Nietzsche also admired the French moralists of the 17th century such as La Rochefoucauld, La Bruyère and Vauvenargues, whose books he received from his sister in 1869. He also admired Pascal and, most of all, Stendhal.See for example Ecce Homo, "Why I am So Clever", §3 He also read Eduard von Hartmann's "Philosophy of the Unconscious", and alludes to it in some of his works.
Nietzsche read in 1883 Paul Bourget's Essais de psychologie contemporaine, from which he borrowed the French term décadence.
Johan Grzelczyk, http://www.hypernietzsche.org/navigate.php?sigle=jgrzelczyk-4 "Féré et Nietzsche : au sujet de la décadence", HyperNietzsche, 2005-11-01 . Grzelczyk quotes Jacques Le Rider, Nietzsche en France. De la fin du XIXe siècle au temps présent, Paris, PUF, 1999, pp.8-9
Bourget had an organicist conception of society. Nietzsche had already encountered organicist theories in Rudolf Virchow's Die Cellularpathologie (1858) and in Alfred Espinas's Des sociétés animales (1887; Die thierischen Gesellschaften, Braunschweig, 1879).
Johan Grzelczyk, http://www.hypernietzsche.org/navigate.php?sigle=jgrzelczyk-4 "Féré et Nietzsche : au sujet de la décadence", HyperNietzsche, 2005-11-01 . Grzelczyk quotes B. Wahrig-Schmidt, "Irgendwie, jedenfalls physiologisch. Friedrich Nietzsche, Alexandre Herzen (fils) und Charles Féré 1888" in Nietzsche Studien, Band 17, Berlin: Walter de Gruyter, 1988, p.439
Nietzsche also became familiar with Darwinism through his early reading of Friedrich Albert Lange's Geschichte des Materialismus (1865), which criticized Darwin's gradualism.
http://fr.wikisource.org/wiki/Note_sur_Nietzsche_et_Lange_:_%C2%AB_le_retour_%C3%A9ternel_%C2%BB Note sur Nietzsche et Lange : « le retour éternel », Albert Fouillée, Revue philosophique de la France et de l'étranger. An. 34. Paris 1909. T. 67, S. 519-525 (on French Wikisource)
Lange alluded to Stirner in this book, whom he (incorrectly) identified with Schopenhauerian positions.Chapter I of Stirner and Nietzsche by Albert Lévy (Paris, Alcan, 1904); for the discussions of a possible influence of Stirner see: http://www.lsr-projekt.de/poly/ennietzsche.html Bernd A. Laska: Nietzsche's initial crisis. (German original in: Germanic Notes and Reviews, vol. 33, n. 2, fall/Herbst 2002, pp. 109-133)
He also mentioned Blanqui's http://classiques.uqac.ca/classiques/blanqui_louis_auguste/eternite_par_les_astres/eternite_.html L'Eternité par les astres, which discussed the thesis of an eternal return. Besides Lange, he read the anti-Darwinist botanist Karl Wilhelm von Nägeli's Mechanisch-physiologische Theorie der Abstammungslehre (1884) in the period of Beyond Good and Evil, which became his main source concerning physiology. Nietzsche targeted Social Darwinism, in particular Herbert Spencer, John Stuart Mill and David Strauss (he read all of them, and titled the first Untimely Meditation "David Strauss: the Confessor and the Writer").
Anette Horn, http://users.iafrica.com/h/ho/hornpete/Nietzsche-Conference.htm "Nietzsche's interpretation of his sources on Darwinism: Idioplasma, Micells and military troops"
Finally, Wolfgang Müller-Lauter showed that Nietzsche also read the embryologist Wilhelm Roux.
Wolfgang Müller-Lauter, "Der Organismus als Innerer Kampf. Der Einfluß von Wilhelm Roux auf Friedrich Nietzsche" in Nietzsche Studien, Bd. 7, 1978, p.189-223
In a letter of 26 February 1888 to Peter Gast, Nietzsche mentions his reading of the posthumous works of Charles Baudelaire (published in 1887). He also read Tolstoy's My Religion (Paris, 1885), the Jewish historian Julius Wellhausen on Arab antiquities and his Prolegomena zur Geschichte Israels (Berlin, 1882), the first volume of the Journal of the Goncourt brothers, thoughts of Benjamin Constant on German theater, Ernest Renan's Life of Jesus — whom he opposed —, and Dostoevsky's The Possessed (Paris, 1886 — read in 1887). Julius Wellhausen became famous for his critical investigations into Old Testament history and into the composition of the Hexateuch, the uncompromising scientific attitude he adopted in testing its problems bringing him into antagonism with the older school of biblical interpreters. He became arguably best known for the Documentary hypothesis on the origin of the Pentateuch. Wellhausen influenced Nietzsche in his writing of The Antichrist and in his musings on the internal discrepancies of the Bible.
Nietzsche's 1888 notebooks also contain references to Victor Brochard's Les Sceptiques grecs (1887); to Charles Féré, who had concerns about "degeneration" issues; and to Louis Jacolliot's Les Lois de Manou, which became for Nietzsche the "classical [case] of pia fraus, the pious lie of religion"
Mazzino Montinari, "La Volonté de puissance" n'existe pas, Editions de l'Eclat, 1996 (§13)
In his notebooks, Nietzsche copied several passages of Féré, later included, without quotation marks, in The Will to Power published by Elisabeth Förster-Nietzsche and Peter Gast.
Johan Grzelczyk, http://www.hypernietzsche.org/navigate.php?sigle=jgrzelczyk-4 Féré et Nietzsche : au sujet de la décadence, HyperNietzsche,2005-11-01
Nietzsche was also an admirer and frequent reader of Ralph Waldo Emerson. Among the German poets, he greatly admired and mentioned in his works Friedrich Hölderlin and Heinrich Heine