There's a good chance that you are dealing with an insecure or scared dog. You should learn about triggers, thresholds, counter conditioning and desensitization. It is important that you don't allow this dog to continue to practice inappropriate behaviors and you should accomplish this without punishment. Your dog is also a resource guarder and needs to learn the game of trading something it is guarding for something better that you have to offer him. Do this enough, and also return what they originally had and the dog will learn that letting you take what they have is a double good idea.
Check out this site for more info about the techniques mentioned above
Working with a fearful, scared or shy dog
There is also a page for books. You should look for one called MINE! as well as reading some of the books focusing on scared dogs.