The Natural History (Latin: Naturalis Historia) by Pliny the Elder, was dedicated to the Emperor Titus in 77 CE, and published posthumously, following Pliny's death while observing the eruption of Vesuvius in 79 CE. It is an encyclopedic work, designed to illustrate a philosophical belief in a benign providence, and to assimilate Greek knowledge for the Roman World. Despite a reputation for credulity, the sheer breadth of Pliny's interests make the Natural History a work of lasting value.
Book I provides a summary of contents and list of Roman and foreign authorities cited. Book II focuses on cosmology and physics, while books III-VI cover the geography of the known world. Book VII deals with the human body, Books VIII-XI with animals, Books XII-XIX with botany. The medicinal properties of plants are covered in Books XX-XXVII and of animals in Books XXVIII-XXXII. Books XXXIII-XXXVII examine minerology, with the aesthetic properties of stones providing the occasion for an excursus on art.
Internet Archive: The Elder Pliny's Chapters on the History of Art, translated by K. Jex-Blake. Multiple formats.
LacusCurtius. Natural History. Latin text, HTML format.
Latin Library: Naturalis Historiae. Latin text, HTML format.
Loebulus. L393 - Pliny -- Natural History VII: Books 24-27. Index of Plants. PDF of public domain Loeb edition in Greek and English. Also at the Internet Archive.
Loebulus. L418 - Pliny -- Natural History VIII: Books 28-32. Index of Fishes. PDF of public domain Loeb edition in Greek and English. Also at the Internet Archive.
University of Chicago: The Historie of the World, translated by Philemon Holland (1601). HTML format.
BBC In Our Time: Pliny's Natural History - Radio programme presented by Melvyn Bragg, with Serafina Cuomo, Aude Doody and Liba Taub.
Librivox: The Natural History - public domain audiobook.
Livius: Pliny the Elder, Natural History.
Roger Pearse: The manuscripts of Pliny the Elder’s “Natural History”.
Wikipedia: Natural History (Pliny).
The Great Conversation: Further reading at Tom's Learning Notes
Pliny is unusual among ancient writers in explicitly citing a copious range of sources. The extant authors mentioned in the summary of the work in Book I include:
Aeschylus - Anacreon - Aristotle - Callimachus - Cato the Elder - Cicero - Columella - Cornelius Nepos - Herodotus - Hesiod - Hippocrates - Homer - Horace - Livy - Menander - Ovid - Pindar - Plautus - Polybius - Pomponius Mela - Seneca - Sophocles - Theophrastus - Thucydides - Varro - Virgil - Vitruvius.
Pliny the Younger: Letters.
Isidore of Seville: Etymologies - An encyclopedic work from late antiquity that relies heavily on Pliny.