Perhaps the most famous fencing treatise of the 1600s, Ridolfo Capoferro's Gran Simulacro is a wonderful book, and an essential read for all fencing scholars. Characteristically, he spells his name as both Capoferro, and Capo Ferro, in the book itself; just one of it's many interesting quirks!
It covers fencing theory, rapier alone, rapier and dagger, rapier and cloak, and rapier and shield.
The download is the CBZ formatted version. You can also get the pdf on this site (but the platform won't let me add this version to the previous product listing, because it's free or pay what you want, and there are file size limits in place).
The book is free, but you are welcome to drop some money into the (virtual) tin; once the book has raised enough money to pay for production costs, we will gladly produce an affordable printed facsimile. Please note that this book is in Italian!