The Library of the Institute of Economic and Social History at the University of Cologne has a web site that links to an enormous number of digital resources, in particular for German history. I find the site a little difficult to navigate, and it looks like the main page may not have been updated in a while. However, I had a great time browsing through this subpage of digital texts, and it seems that new texts are actively being added there. I was excited to find Künste und Geheimnisse großer Künstler, zum Gebrauch der Mahler, Bildhauer, Schwerdfeger, Uhrmacher, Buchbinder, Schreibemeister, u. dergl., published in Prague and Leipzig in 1771. The book is in two parts, and the scanned pages can be viewed as PDF files, in 10-page chunks. I haven’t had a chance to look through it very carefully, but what seems interesting is that it’s a very practical, how-to book. It seems like it could be a good reference for old painting techniques, especially.