Great Sausage Recipes and Meat Curing by Rytek Kutas. This is the guy that runs The Sausage Maker. A lot of the members of this site believe that if you are only going to have one book on sausage, this is the one to have.
I checked this book out plus all the other books on the subject of sausage making & charcuterie at our local library. Of the 5 or 6, this one was by far the best. I'm going to put this book on my Christmas/birthday list so I have my own copy.
I also have Charcuterie: The Craft of Salting, Smoking, and Curing by Michael Ruhlman and Brian Polcyn. These two books will go a long way toward teaching you everything you need to know about making, curing, and smoking sausage, plus a lot of great recipes. I usually start by following the recipes as closely as possible, then try adjusting according to my own tastes the next time around. I take notes and like to weigh all my ingredients so I know exactly what goes in. I find that especially when it comes to spicy sausage that the recipes are on the mild side.