For understanding Luther’s interpretation of the Bible, following items might be worth consulting. The list does not contain modern editions, which include many patristic and medieval authors such as Augustine, Jerome or Gregory the Great.
Augustine‘s works, the volumes containing biblical commentaries. 1506 Basel edition, which was also available in Wittenberg, has been digitized by Bayerische Staatsbibliothek, München and Thüringer Universitäts- und Landesbibliothek, Jena (collection “Bibliotheca Electoralis”, including the former library of the University of Wittenberg). Includes commentaries of Francis of Meyronnes.
http://archive.thulb.uni-jena.de/hisbest/receive/HisBest_cbu_00004525 (not yet digitized)
http://archive.thulb.uni-jena.de/hisbest/receive/HisBest_cbu_00004420 (not yet digitized)
http://archive.thulb.uni-jena.de/hisbest/receive/HisBest_cbu_00004419 (not yet digitized)
Vol. 5 (De Genesi ad litteram libri XII, cum conclusionibus sive veritatibus Francisci Maronis):
http://archive.thulb.uni-jena.de/hisbest/receive/HisBest_cbu_00004418 (not yet digitized)
Vol. 7 (De civitate Dei)
http://archive.thulb.uni-jena.de/hisbest/receive/HisBest_cbu_00004414 (not yet digitized)
Vol. 9 (In Euangelium secundum Johannem; In epistolam canonicam Sancti Johannis; In Apocalypsim):
Vol. 10 (De mirabilibus sacrae scripturae libri III cum conclusionibus sive veritatatibus Francisci Maronis; De eo quod dictum sit: Ego sum qui sum cumconclusionibus sive veritatibus Francisci Maronis):
Vol. 11 (Quaestionum veteris et novi testamenti liber unus; Questionum evangeliorum secundum Matthaeum liber unus; Super Magnificat canticum Mariae expositio):
Bible published by Sebastian Brant in Basel 1498, with commentaries of Nicholas of Lyra, Paul of Burgos and Matthias Döring, digitized by Bayerische Staatsbibliothek, München.
Bible, with postil of Hugh of Saint-Cher (Hugo Cardinalis)
Quincuplex Psalterium by Jacques Lefèvre d’Étaples (Faber Stapulensis) has been digitized by Bibliothèque nationale de France:
Rudimenta linguae Hebraicae by Johann Reuchlin has been digitized by Bayerische Staatsbibliothek, München:
Several works of Erasmus of Rotterdam are also available, see for example: http://www.philological.bham.ac.uk/bibliography/e.html (Erasmus, Desiderius). Among these are the Annotations to the New testament in the 1538-40 edition of Erasmus’ Opera omnia, digitalized by the Erasmus Center for Early Modern Studies in their Erasmus database in http://www.erasmus.org/index.cfm?fuseaction=eol.searchform (keyword: Erasmus, click on “Show facsimiles only”).