Manuals

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. 

Longsword

I like Hugh Knight's 'Knightly Art of Longsword' as a entry point into reading an interpretation of longsword fencing. The material has held up very well over my years of practice, and he explains material that is otherwise very confusing in the original manuals. This book makes for an excellent companion to the translations on Wiktenauer and is fairly well annotated.

Langes Messer

Wiktenauer has a good number of translation of Lecküchner's treatise on the Langes Messer. You can get a printed version (which is nice but does not have the original text and has a couple issues, but I overall recommend). Lecküchner’s 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 reads more like an encyclopedic index of techniques rather than a comprehensive fencing manual.

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.