The Roman History (original Latin title: Res Gestae A Fine Corneli Taciti) of Ammianus Marcellinus, amongst the last major historians of the ancient world, originally covered the period from the end of Tacitus' Histories in around 96 AD to to the Battle of Adrianople in 378 AD. Only the section from 353 to 378 AD is extant. Marcellinus recounts the struggles of the late Roman Empire against mounting threats on its many borders, as well as internal sectarian strife, writing from a tolerant Pagan perspective.
Free Online Texts
Gutenberg: The Roman History of Ammianus Marcellinus. Multiple formats.
Tertullian.org: Ammianus Marcellinus - English HTML text.
Wikisource: Roman History.
Internet Archive: The text tradition of Ammianus Marcellinus by Charles Upson Clarke (1904). Multiple formats.
Latin Library: Ammianus Marcellinus. Latin text, HTML format.
Radboud University: Ammianus Marcellinus - Bibliography by Marc Van Der Poel.
Rijksuniversiteit Groningen: Ammianus Marcellinus Online Project.
Wikipedia: Ammianus Marcellinus.
The Great Conversation: Further reading at Tom's Learning Notes
Tacitus: The Histories - This work ends around where the Res Gestae originally began, effectively making Ammianus' work a continuation.