I bought a book today about Da Vinci and it's pretty good so far and I also bought the new American Artist Drawing magazine that just came out, that has in some about Renaissance artists and their materials and Techniques. In it was also link to a gallery of Da Vinci's drawings which are pretty nice.
There is also another great link to a bunch of other masters, which you can zoom in and out of. There is tons of artists on this site under the artchive section, it's kinda like artrenewal.
I almost forgot I had this cool link to Da Vinci's notebooks. It's a flash program that you can actually turn the pages and zoom in and out. You can find it here: