The New Essays on Human Understanding (French: Nouveaux essais sur l'entendement humain) by Gottfried Wilhelm Leibniz, is a response to Locke's Essay Concerning Human Understanding, originally completed in French in 1704, but withheld from publication after Locke's death until 1765.
The New Essays provide a detailed critique of Locke's work in dialogue form, with one speaker Philalethes, defending Locke's position and a second, Theophilus, presenting Leibniz's own views, including a strong defence of the rationalist doctrine of innate ideas.
Free online texts
Early Modern Texts: New Essays on Human Understanding, adapted and translated into modern English, by Jonathan Bennett. PDF format.
Internet Archive: New Essays Concerning Human Understanding. English translation (1916). Multiple formats.
Wikisource: French text. HTML and other formats.
Gutenberg: Leibniz's New Essays Concerning the Human Understanding: A Critical Exposition, by John Dewey (1902).
PhilPapers: Gottfried Wilhelm Leibniz - bibliography with open access option.
Further reading at Tom's Learning Notes
John Locke: An Essay Concerning Human Understanding.