To be quite honest, I just experiment a bit with stuff, sometimes you get a bit hungry when it goes wrong though! I have Huge Furry Whitllingtools Cook on the Wild Side, and it is a good book to learn what you can use from hedgerows and the like. He cooks them at home though, but you just apply knowledge from home cooking and translate that into cooking outdoors. It's best to try and keep things simple, complicated things tend to go wrong easily, which is why I like to try and grill sausages and bacon by putting them onto sticks other than trying to make a greenwood grill to place over the fire. I'm not saying it's wrong or anything, I just like to do things easily without burning too many calories!!
I also get hold of beans in a carton instead of a tin and like to boil them up in the billy with some spices and a handful of rice thrown in for bulk, don't forget to add some water though or it will be very dry and will burn and stick to your billy can. Don't ask me how I know that!!