There are two, global, core reference depositories for all things HEMA related:
If you can’t find it here, it probably isn’t available on the open academic market!
For printed books, the following are recommended reading:
Beginners/Good introduction: AHA-german-longsword-study-guide
The specific, core, German Longsword training treatise used by TLA is the excellent book by Michael Chidester et al. It is available to download from http://hroarr.com/manuals-books/new-manual-section/?did=30