Lentils and split peas are excellent muscle food. Both are very easy to prepare, cheap, and quite tasty. They also happen to have the lowest carb:fibre ratio of the commonly available beans, which is a very good thing. You can make a number of different, very tasty soups that are high in protein and fibrous carbs by simply throwing some kind of lean meat (after browning, of course), either or both types of bean, and whatever vegetables/herbs you like into a pot for an hour or two.