Digital facsimile from The Medical Heritage Library, Internet Archive, at this link. 

One of the first works on the geographical distribution of plants. Humboldt was a pioneer student of geographical–ecological plant associations. The sheets of this work were reissued as Vol. I of the authors' Voyage aux régions équinoxiales du nouveau continent, fait en 1700, 1800, 1801, 1802, 1803 et 1804, with an extra half-title and general title and the plate colored. Digital facsimile of the 1805 edition from the Internet Archive at this link. English translation: Essay on the geography of plants, edited with an introduction by Stephen T. Jackson, translated by Sylvie Romanowski. Chicago: University of Chicago Press, 2009.

In 1799 Humboldt and Bonpland embarked on a six-year tour of research through South America and Mexico, a trip which would afterwards be called, justifiably, "the scientific discovery of America."  The two amassed exhaustive data in a wide array of fields from meteorology to ethnography, and gathered 60,000 plant specimens, 6,300 of which were previously unknown in Europe. Their American travel journals— issued under the general title Voyage aux régions équinoxiales du nouveau continent, fait en 1799, 1800, 1801, 1802, 1803 et 1804— were published in thirty-four volumes between 1807 and 1834. Digital facsimiles are available from the Internet Archive and Bnf Gallica. 

Digital facsimile from Biodiversity Heritage Library at this link.

Matteucci’s “rheoscopic frog” effect.

This work includes founding material in geography, plant geography, orography, climatology and meterology, among other sciences, and citations to 9,000 sources. The atlas was issued without Humboldt's participation, and published by Krais & Hoffmann (1851). The fifth volume comprising the lengthy index, was issued after Humboldt's death. 

There were several English translations: by A. Prichard in 2 vols., London: H. Baillière, 1845; by Mrs. Eliz. Sabine, 4 vols, London: Longmans, 1846-58, and the most complete in 5 vols. by E. C. Otté (vols 1-3), Otté and B. H. Paul (vol. 4), and Otté and W. S. Dallas (vol. 5), London, 1849-52. The best introduction to the work is that to Vol. 1 by Nicolaas A. Rupke, Cosmos: A Sketch of the Physical Description of the Universe, Baltimore: Johns Hopkins University Press, 1997. There are numerous digital facsimiles available.