Mediaevum
This site has a number of links to various Medieval German dictionaries: Mittelhochdeutche; Mittelniederdeutsches, Althochdeutsch, Gotisch, Neuhochdeutch, and more.
Mediaevum: Authors and Works of the German Middle Ages
Offered here is very useful information about key literary authors and their texts, collections of digitized texts, and even links for the purchase of books.