I learned a lot from this book, Bruce Aidells's Complete Sausage Book : Recipes from America's Premium Sausage Maker. Haven't bought a copy yet, as I'm working through suasage info in my present books. I have an older copy of the book CC mentions too.
This site, while not the best, is part of a web-ring, a series of different sites on the same topic. At the bottom of the page is a box with a series of links to the next sites. The home page for this specific web-ring is down, but still plenty of good info to explore.http://www.exit201.com/sausstart.htm
My bro-in-law killed a pig recently. In the side I bought, I had the butcher package up the trim and fat in 2 pound packages for me to make into sausage at my leisure. I made some breakfast style sausage two days back. The package was close to 1/3 fat, more than I wanted, but much more than you get in commercial sausage anymore. Turned out a tender and flavorful sausage with real drippings for the biscuit and gravy and sausage breakfast.
The butcher rendered out an amazing amount of lard from the pig too, a very fat pig. I trimmed and trimmed on the ham to get it down to the fat I wanted and it was still the fattiest ham I had in a long time. Excellent flavor though.