The Anabasis of Alexander (Greek: Ἀλεξάνδρου Ἀνάβασις) is an account of the conquests of Alexander the Great by Arrian, a Greek historian of the second century CE. The title reflects the influence of Xenophon's Anabasis on its structure and approach.
Arrian's work is the fullest extant account from the ancient world of Alexander's campaigns, drawing on contemporary sources which are now lost.
Free online texts
Perseus: Anabasis Alexandri, Greek text, edited by A.G. Roos (Teubner 1907).
Wikisource: The Anabasis of Alexander; or, The history of the wars and conquests of Alexander the Great. (1884) by Arrian, translated by E. J. Chinnock. Online, downloadable via Bookcreator. Greek text: Αλεξάνδρου Ανάβασις.
Wikipedia: The Anabasis of Alexander
The Great Conversation: Further reading at Tom's Learning Notes
Xenophon: Anabasis - An account of an earlier Greek expedition to the Persian Empire which may have influenced Alexander's campaign as well as Arrian's account of it.
Curtius: Histories of Alexander the Great - a somewhat uncritical Latin account.
Plutarch: Parallel Lives - Alexander the Great is paired with Julius Caesar.
Ancient Greek resources: Learn to read Greek classics in the original.