The Church of the West in the Middle Ages Volume 1: From Gregory the Great to St. Bernard 590-1153 (English Edition)

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This is a history of Christian Church in the centuries after the death of Christ, and the way in which it spread across Europe.
From the preface:
“AUTHORITIES.—The student should read and re-read J. BRYCE’S Holy Roman Empire. Of the argument (early editions) a useful summary is given by E. FREEMAN, ESSAYS, ii. For the later theory of the Papacy and Empire, see R. L. POOLE, Illustrations of Medieval Thought, c. 8; FISHER, Medieval Empire, a scholarly work, supplements Bryce (stiffer reading). A good historical atlas is indispensable. That of the Clarendon Press will probably be too dear for most; PUTZGER’S Historischer Schul-Atlas is a marvel of accuracy and value.
Additional aids to study are the following:—T. HODGKIN, Italy and her Invaders, v. and vi. (vol. v. c. 7 contains a full account of Gregory’s life and work). F. HARRISON, two essays on the Eastern Empire in his Meaning of History. See also on the same subject, FREEMAN, Essays, v. iii. Add T. HODGKIN, Charles the Great. J. B. BURY’S Gibbon, c. 49, espec. vol. v. App. 16.
For sources of Gregory’s life, see BURY, op. cit. v. 510, F. W. KELLETT’S Pope Gregory the Great and his Relations with Gaul is a scholarly monograph. The best English life of Gregory and his times is F. H. DUDDEN, Gregory the Great. For the supposed letters of Gregory II., see HODGKIN, op. cit. vi. 501, or BURY, l.c. v. App. 14. They are, however, accepted by Milman and Gregorovius (wrongly, I think).
For the lives of the early popes the student is largely dependent on the scholarly edition of the Liber Pontificalis by the Abbé DUCHESNE (French).”