The Historical Library (Greek Βιβλιοθήκη ἱστορική) is a universal history written in the mid-first century BCE by Diodorus Siculus, a Sicilian Greek. The surviving portions of the work are Books I-V, which examine the early, often legendary history of the verious parts of the known world in turn, and books XI-XX, which provide a continuous historical narrative from 480 to 302 BCE. Although often derivative, Diodorus is important for those periods when he is the main surviving source.
Free online and downloadable texts
Attalus.org: Historical Library: Books 33-40.
LacusCurtius: The Library of History.
Perseus: Diodorus Siculus. Diodorus of Sicily in Twelve Volumes with an English Translation by C. H. Oldfather. Vol. 4-8. Cambridge, Mass.: Harvard University Press; London: William Heinemann, Ltd. 1989. Online text. Greek text at this link.
Theoi.com: Library of History, Book IV. Online text of the Oldfather translation.
Wikisource: Historical Library, translated by George Booth, 1814. Online text, downloadable via Bookcreator.
Livius: Diodorus of Sicily.
Wikipedia: Diodorus Siculus
Tertullian.org: Diodorus Siculus: the Manuscripts of the "Bibliotheca Historica".
The Great Conversation: Further reading at Tom's Learning Notes
Polybius: The Histories - a key source for the third decade of the Historical Library.
Ancient Greek resources: Learn to read Greek classics in the original.