Congratulations on your new dog! What a wonderful person you are to take in an older dog. Not many people are willing to do that unfortunately. I think bckrazy's advice is excellent. However, I do think your dog is being protective. My dog Duncan is overprotective and behaves in much the same manner except he does lunge at strangers and would bite them if given the chance. I have tried everything with him, including 3 different very expensive one-on-one trainers. Nothing has changed. It's a difficult situation because it keeps circling back on itself...You take the dog for a walk and a stranger comes walking toward you, the dog barks, you tell him no but since you can't take the chance of the dog biting the stranger, you keep the dog close to you and away from the passer-by, you get nervous everytime someone approaches, increasing the dog's idea that there is something to fear, the dog becomes more aggressive to strangers because he/she is being "taught" to avoid them....It's a vicious circle.
I would definitely advise getting professional help. Or finding a really good friend that the dog has yet to meet and practicing walking by them over and over again until the dog and yourself, no longer react. You may want to do this with a muzzle on the dog in case the dog does try to bite your friend. The dog's life and safety come first.
I live with this situation everyday and it's very difficult. The dog may be feeling insecure since he lost his/her home of 9 years and feels that if he/she loses you, what then? Good luck and keep us posted!!

Oh and welcome to Pet Talk!!