Do you mean Art, or Artillery? If you're talking about Handgonnes of the medieval age, there's a great site about the subject here: http://homepages.ihug.com.au/~dispater/handgonnes.htm Look for the upcoming Osprey book on that subject.

If you mean the larger Culverins, Bombards and the like, allow me to direct you here: http://xenophongroup.com/montjoie/guns_etc.htm

As well as http://www.sfusd.edu/schwww/sch618/war/cannon.html nice pictures but the text is sparse. Osprey (again) has a very good and colorful book MEDIEVAL SIEGE ARTILLERY which is worth checking out.