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.
Vol. 1: http://mdz10.bib-bvb.de/~db/0002/bsb00026023/images/
Vol. 2: http://mdz10.bib-bvb.de/~db/0002/bsb00026025/images/
Vol. 3: http://mdz10.bib-bvb.de/~db/0002/bsb00026026/images/
Vol. 4: http://mdz10.bib-bvb.de/~db/0002/bsb00026029/images/
Vol. 5: http://mdz10.bib-bvb.de/~db/0002/bsb00026030/images/
Vol: 6 http://mdz10.bib-bvb.de/~db/0002/bsb00026033/images/
Bible, with postil of Hugo de Sancto Caro (Hugo Cardinalis)
Vol. 1: http://mdz10.bib-bvb.de/~db/0002/bsb00026098/images/
Vol. 2: http://mdz10.bib-bvb.de/~db/0002/bsb00026099/images/
Vol. 3: http://mdz10.bib-bvb.de/~db/0002/bsb00026100/images/
Vol. 4: http://mdz10.bib-bvb.de/~db/0002/bsb00026101/images/
Vol. 5: http://mdz10.bib-bvb.de/~db/0002/bsb00026102/images/
Vol. 6 missing
Vol. 7: http://mdz10.bib-bvb.de/~db/0002/bsb00026105/images/
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”).