The Church History (Greek: Ἐκκλησιαστικὴ ἱστορία; Latin: Historia Ecclesiastica or Historia Ecclesiae) of Eusebius was the first major historical work written from a Christian point of view. It details the history of the church from the time of Christ down to the early fourth century AD. As Bishop of Caesarea, had access to many earlier sources which are quoted in the work.
Free online and downloadable texts
Archive.org: Church History. Life of Constantine the Great. Oration in Praise of Constantine. Multiple formats.
Christian Classics Ethereal Library: Eusebius Pamphilius: Church History, Life of Constantine, Oration in Praise of Constantine. From Nicene and Post-Nicene Fathers, Second Series, Vol. 1. Edited by Philip Schaff and Henry Wace. (Buffalo, NY: Christian Literature Publishing Co., 1890.) . Multiple formats.
Loebulus. L153 - Eusebius - Ecclesiastical History I: Books 1-5. PDF of public domain Loeb edition in Greek and English.
Loebulus. L265 - Eusebius -- Ecclesiastical History II: Books 6-10. PDF of public domain Loeb edition in Greek and English.
New Advent: Church History (Eusebius). Translated by Arthur Cushman McGiffert. From Nicene and Post-Nicene Fathers, Second Series, Vol. 1. Edited by Philip Schaff and Henry Wace. (Buffalo, NY: Christian Literature Publishing Co., 1890.) Revised and edited for New Advent by Kevin Knight.
Documenta Catholica Omnia: Eusebius Caesariensis Historia ecclesiastica - Greek text only.
Librivox: Eusebius History of the Christian Church - public domain audiobook.
Wikipedia: Church History: Eusebius.
The Great Conversation: Further reading at Tom's Learning Notes
Ancient Greek resources: Learn to read Greek classics in the original.