I am baking bread, and the recipe (from the Bread Bakers Apprentice) calls for shortening. I don't have any in the house and would like to substitute butter. The recipe does not indicate that the shortening needs to be liquid. Should I melt the butter that I will be using in place of the shortening?
No, shortening is a solid fat. This means you have to substitute another solid fat, else the recipe won't work. So, use the butter as it is.
You probably will have to bring the butter to room temperature to be workable (shortening hardens less in the fridge). Don't use the microwave, it will produce melted spots. Leave it out overnight or longer, or, in the worst case, shave it into small sheets and leave these out for an hour or so.
Your texture will differ somewhat from the original. Lard might be the better substitute for shortening.