The following are links to Medieval Church History primary source documents available online, in English translation, for free.  In some cases, there are better translations available in print, so for serious scholarly publications, you will want to consult those.  But for introductory purposes, the following links may be helpful.

General Medieval History
•  Frederic Austin Ogg, ed., A Sourcebook of Medieval History (1907) (Google book) - some excerpts simplified
•  Ernest Henderson, ed., Select Historical Documents of the Middle Ages (1892) (Google book)
•  James Harvey Robinson, ed.,  Readings in European History: From the Breaking Up of the Roman Empire to the Protestant Revolt, Volume 1 (1904) (Google book)
• Oliver J. Thatcher, ed.,  A Source Book for Mediaeval History (1905) (Google book)

• Medieval Sourcebook (Fordham) includes links for the following categories
          • Saints' Lives
          • Medieval Spiritual Writing

• Medieval authors on Post-Reformation Digital Library

• Classics of Western Spirituality - 120 volumes - searchable on google books - for complete list of volumes -

Medieval Individuals

• Abelard, Peter and Heloise
      • Lives and Letters of Abelard and Heloise (archive.org)
      • Love Sonnets of Abelard and Heloise (archive.org)
      • Story of My Misfortunes (archive.org)

• Albertus Magnus
     •  On Union with God (archive.org)
     •  (His Latin works)

• Anselm of Canterbury
     •  Meditations and Prayers (archive.org)
     •  Basic Writings (Proslogium, Monologium, Baunilon's: On Behalf of the Fool; Cur Deus Homo) - (archive.org)

     •  On Truth (jasper hopkins: 2000 edition)

     •  Freedom of Choice (jasper hopkins: 2000 edition)

     •  The Fall of the Devil (jasper hopkins: 2000 edition)

     • The Incarnation of the Word (jasper hopkins: 2000 edition)

     • Of the Virginal Conception (jasper hopkins: 2000 edition)

     • Of the Procession of the Holy Spirit (jasper hopkins: 2000 edition)

     • Of the Harmony of God's Foreknowledge and Choice (jasper hopkins: 2000 edition)

• Aquinas, Thomas (1225-1274)
          • Commentary on Aristotle's Treatise of the Soul - (archive.org)
          • Summa Theologica (ccel.org) and Summa Theologica (new.advent)
          • Summa Contra Gentiles - somewhat abridged (google pdf: 1905 edition)
          • On Being and Essence - (medieval sourcebook)

          • On the Principles of Nature (isidore.co)

          • Catena Aurea - with parallel English translation (dhspriory.org)
          • Bibliography of Aquinas' works in English - includes links to online sources (duq.edu)

          • Aquinasonline.com

        
• Bernard of Clairvaux
     • On Loving God  (ccel.org)
     • Letters (google book pdf:  1904 edition; 1889 edition: Volume 1; Volume 2)
     • On Consideration (google book pdf: 1908 edition)
     • Life of St. Malarchy (google book pdf: 1920 edition)
     • 86 Sermons on Song of Solomon (google book pdf: 1896 edition)

• Cell of Self-Knowledge - 1521 (archive.org) - 7 English mystical treatises

• Catherine of Siena (1347-1380) - Italian mystic, theologian
          • Letters - (gutenberg.org)
          • Dialogue - (archive.org)
          • Life - by her confessor, Raymond of Capua - (archive.org)

• Clare of Assisi

          • Rescript granting Holy Poverty to St. Clare (franciscan-archive.org)

• Conrad of Saxony
     • Mirror of the Blessed Virgin Mary - formerly attributed to St. Bonaventure (intratext.com)

• Dante
          • Divine Comedy 

• Francis of Assisi
          • The Testament (ofm.org)

          • Letters to a General Chapter (franciscan-archive.org)

          • On the Qualities of the Minister General (franciscan-archive.org)

          • On the Devotion the Friars should have for the Holy Rule(franciscan-archive.org)

          • Canticle of the Sun (webster.edu)

• Guibert of Nogent (d. 1124)
     • Autobiography (medieval sourcebook)

• Hildegard of Bingen

    • Explanation of Athanasian Creed (academia.edu)

• Hus, Jan
          • Links to Works - (lollardsociety.org)

• Kempe, Margery (c. 1373-c.1438) - English pilgrimage and visionary
          • Middle English version - (d.lib.rochester.edu)

• Lazaros of Mt. Galesion - his Life - (doaks.org pdf)

• Lombard, Peter
          • The Sentences -
          • Letters - (google pdf: 1912 edition)

• Manipulus Florum - Collection of quotations for use in medieval preaching (the-orb.net)

• Middle English Texts (for more, see Corpus of Middle English Prose and Verse)

     • The Book of Curtesy (EETS on google books)

     • Chaucer's Translation of Boethius' Consolations of Philosophy (EETS 5 on google books)

     • Gesta Romanorum (EETS 33 on google books) 

     • George Ashby's Poems (EETS 76 on google books)

     • Godfrey of Boloyne or the Siege and Conquest of Jerusalem (EETS 64 on google books)

     • Harrowing of Hell and Gospel of Nicodemus (EETS on archive.org)

     • The Northern Passion (EETS on google books)

     • The Life and Death of Mary Magdalene (EETS 78 on )

     • Lydgate's Temple of Glas (EETS 60 on google books)

     • Lydgate's Siege of Thebes (EETS 108 on google books)

     • Mirk's Festial: A Collection of Homilies (EETS 96 on google books)

     • The Poems of William of Shoreham (EETS 86 on google books)

     • Supplication for the Beggers (EETS 13 on google books)

     • Three Middle English Sermons (Leeds School of English Language on archive.org)

• Public Record of Labor of Isabel de la Cavalleria (the-orb.net)

• Wyclif, John
          • Links to Works - (lollardsociety.org)

          • Select English Works: Sermons on the Ferial Gospels and Sunday Epistles; Treatises - (googlebooks: volume 2; 1869 edition)

          • Tracts and Treatises (googlebooks: 1845 edition)

          • English Works (googlebooks: 1880 edition)

          • Bible Transltation (googlebooks: 1879 edition)

Medieval Topics

• Art
          • Early Medieval - (arthistoryresources.net)
          • Gothic - (arthistoryresources.net)
          • Renaissance in Italy - (arthistoryresources.net)
          • Renaissance outside of Italy - (arthistoryresources.net)

• Books of Hours
       • Hypertext version of 1599 Book of Hours text - (medievalist.net)
       • Book of Hours of the Virgin (1599)
       • Book of Hours of Yolande of Flanders (14th C) (google book pdf: 1909 edition)
       • 32 Minatures from Book of Hours of Joan II, Queen of Navarre (14th C) (google book pdf: 1899 edition)
       • Book of Hours attributed to Hans Memling (15th C) - (google book pdf: 1908 edition)
       • Book of Kells' illustrations - c. 800 A.D. (google book pdf: 1920 edition)(digitalcollections)
       • A Parisian Book of Hours - illustrations - (archive.org)
The following texts and sites include Latin manuscripts whose illuminations may be used as primary sources
       • Books of Hours - full color illustrations of all pages within books for sale
       • Book of Hours - 1524 (Library of Congress)
       • Book of Hours - Cistercian, late 13th C (world digital library)
       • Book of Hours of the Virgin Mary - Paris, mid-15th C (world digital library)
       • Loftie Hours - Dutch, mid-15th C (world digital library)
       • There are many others on world digital library as well...

• Byzantine/Orthodox
          • Byzantine Monastic Foundation Documents - (archive.org)
          • Holy Women of Byzantium: Ten Saints' Lives - (archive.org)
          • The Philokalia (archive.org)

• Coptic
       • Ibn-Kabar (14C) The Lamp that Lights the Darkness - (coptic.net)

• Crusades
     • Chronicles of the Crusades:  Narratives of the Crusade of Richard Coeur de Lion - (google book pdf: 1848 edition)
     • The Medieval Sourcebook:  Crusades - many excerpts from Crusade sources
     •  Dana Carleton Munro, ed., Letters of the Crusaders (Translations and Reprints from the Original Sources of European History)
     •  Dana Carleton Munro, ed., The Fourth Crusade (Translations and Reprints from the Original Sources of European History) (1896)
     •  James Harvey Robinson, ed.,  The Pre-Reformation Period (1896) (Translations and Reprints from the Original Sources of European History)

• Drama - Mystery Plays
       • Cornish Ordinalia - Volume 1, Volume 2 - (google book pdfs)
       • York Mystery Plays - (google book pdf)
       • Towneley Mystery Plays - (google book pdf)
       • N-Town - modernized version (University of Toronto)
       • Coventry/Corpus Christi
              • Shearmen and Taylors - (google book pdf)
              • Coventry Mysteries - (google book pdf)
       • Chester Plays - Volume 1, Volume 2 - (google book pdfs)


• Lollardy (followers of Wyclif) - Bibliography of Primary Sources (on-line sources marked by red square) - (Lollardsociety.org) - includes additional non-Lollard devotional texts

• Mass / Eucharist
     • Mass of the Roman Rite (Latin/English)
     • The Assyrian Liturgy

• Monasticism
           • Cistercian
           • Charta Caritatis (osb.org)
           • Benedictine Rule - (osb.org)
           • Franciscan Rule - (ofm.org)
          • Ancren Riwle - Rule for anchoresses

• Nestorian/Chinese
       •  Stele (781 A. D.) (Fordham)