Kirsch, Johann Peter. "Anastasius Bibliothecarius." The Catholic Encyclopedia. Vol. 16 (Index). New York: The Encyclopedia Press, 1914. http://www.newadvent.org/cathen/16002b.htm. Accessed 19 Aug 2014.
If a passage in the annals of Hincmar of Reims is genuine (Mon. Germ. Hist.: Scriptores, I, 447) and Hincmarhas not confused two men, then the librarian Anastasius is identical with the Roman presbyter Anastasius who in 874 became titular priest of St. Marcellus, and in 848 fled from Rome, and resided in various cities. On account of his flight he was excommunicated by a Roman synod in 850, and, as he did not return, wasanathematized and deposed by another synod in 853. After the death of Leo IV in 855 this Anastasius was elected as antipope by the imperial party, but the rightfully elected pope, Benedict III, gained the supremacy, and acted kindly towards the usurper. During the pontificate of Adrian II Anastasius became involved in serious difficulties, in 868 a near relative of his named Eleutherius forcibly carried off the daughter of the pope, and soon after killed both her and her mother. The murderer was executed and Anastasius, who was regarded as the instigator of the murder, was punished by excommunication and deposition. He lived at the imperial Court, and sought by the intervention of the emperor to exculpate himself before the pope. Hergenröther (Photius, II, 230-240) maintains, with good reason, that the librarian and the presbyter Anastasius (the antipope) were one and the same person, and weaves all the statements concerning the latter into the biography of Anastasius, while Langen (Geschichte der römischen Kirche, III, 270 sqq.) considers them different persons. In August, 879, Zacharias of Anagni appears as librarian of the Roman Church, so that Anastasius must have died shortly before this date.
Anastasius translated from Greek into Latin the "Acts" of the Seventh and Eighth Ecumenical Councils, as well as several legends of saints, and other writings. He also compiled a historical work, "Chronographia tripartita", from the Greek writings of Theophanes, Nicephorus, and Syncellus, and made a collection of documents concerning the affairs of Pope Honorius. Several important letters written by him have been preserved. His writings are to be found in P.G., XXIX; P.L., LXXIII, CXXII, CXXIX. The "Liber Pontificalis", which was formerly ascribed to him, was not written by him; he seems to have shared in the revision of the "Life" of Nicholas I.
LAPOTRE, De Anastasio Bibliothecario (Paris, 1884); KRUMBACHER, Geschichte der byzantinischen Litteratur, 2nd ed., 344 sq.; FRIEDRICH, Ein Brief des Anastasius Bibliothecarius an den Bischof Gaudericus von Vellevri in Sitzungsber. der bayer. Akademie der Wiss. phil.-hist. Klasse (1892-93), 393 sqq.; D'AVRIL, La lettre d'Anastase le Bibliothecaire in Revue de l'Orient chretien (1896), 124 sqq.