Fechtbuch is really a martial arts manual, often illuminated with pictures of the fighting maneuvers, meant as a teaching tool. A sort of spell book for fighters.
You might want to use the concept in your campaigns...
a fighter that finds one might reach a new skill level, class level, specialty, feat, proficiency, etc. Manuals with special techniques would be much sought after, especially if they contained secret winning techniques.
Or you might wonder a bit at a few translated texts from (for example) the Von Danzig manual here. They are provided by Schielhau.org, or Mike Rasmusson, a self-described longsword addict from Toronto. Together with other addicts he made available an impressive set of resources on Fechtbucher.
Image is from the first folio of MS 1449 by Peter von Danzig, by grace of Wikipedia