Due to different digitalization projects, many important texts have now become available for scholars. For understanding Luther’s interpretation of the Bible, following items might be worth consulting:
Bible published by Sebastian Brant in Basel 1498, with commentaries of Nicholas of Lyra, Paul of Burgos and Matthias Döring, digitalized by Bayerische Staatsbibliothek, München.
Bible, with postil of Hugo de Sancto Caro (Hugo Cardinalis)
Vol. 6 missing
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”).