For all of the Fechtbucher (Historical German fightbooks) you can find tons of information on Wiktenauer. I strongly recommend you read some of them, even if it's just to get an idea. These texts are the basis of HEMA, and what the curriculum is built upon. This site is a priceless source of information providing many of the original manuals in translated format for free. They are not especially easy to understand, but having access to such an extensive amount of information is truly valuable beyond compare when studying historical swordsmanship.
Freelance Academy Press also has an impressive selection of books and video related to the German tradition of combat.
I use Hugh Knight's 'Knightly Art of Longsword' as the core of my curriculum. He has an excellent presentation of the material and extensive references so that you can go and find the source material, which is very very useful.
There are other sources I use, but they are mostly auxiliary to Hugh's book.
The Hammaborg site has a wonderful translation of Leckuchner's treatise on the Langes Messer. His work is the earliest and most thorough that I've come across on the subject. Unfortunately this can be a bit difficult to understand since it is primarily a translation rather than an interpretation.
Dagger, Wrestling, and Pollaxe
The rest of Hugh's books are available on the subject and they are quite excellent . I would like to point out the pollaxe and wrestling/dagger books in particular however. These are great interpretations for the subjects, and I highly recommend them if you plan to study these arts.