What to do in this situation?

Next time offer him a treat that is better than the bone. Trying to take that bone away will only make him more grumpy and more aggressive. Hold up the treat and say look what I have, yumm... act happy make whatever you have to give him sound better then what he is holding. He will drope the bone and then he gets the treat. A good "drop it" command is very important.

Unfortunately the word NO doesn't mean much to dogs, and it sure doesn't tell them what you WANT them to do.  Now, look at this from HIS perspective....he found something great and you started yelling NO at him, took it away and then left him in the dark.
 
Anne (spiritdogs) has a GREAT trade protocol.  I'll email her in case she doesn't see your thread.

The problem is that there is nothing better. Also, I think he wants to show us who is boss. So even if we had a better treat, I think he'd keep holding onto the bone.

Does anyone know of any books that deal with behaviour problems of stray dogs? We're not just talking about aggression here. We're talking about a dog that is being defensive after having been mistreated in a previous home.

Quite a few dogs will drop a bone for a piece of liverwurst, or garlic roast beef.  When you offer it, and the dog does drop the bone, you say "trade" and he gets the liverwurst while you pick up the bone (carefully).

Grabbing and yelling only makes a dog guard more aggressively.  Also, when you chase and yell, some dogs try to swallow their "prize", which may be the very thing you are trying to avoid.

In the case of a dog that you are really frightened will bite and not trade, and you are simply trying to avoid injury to the dog or humans, you could opt to throw pieces of cheese, turkey, whatever meat you have, all over the floor so that he has to leave the bone to get the "bonanza".   Dogs care how many pieces of food are strewn about, and will often leave "1" bone to get "20" pieces of meat.

You should get a copy of Jean Donaldson's book, "Mine! A Guide to Resource Guarding in Dogs" and follow the whole training protocol.  This is a problem that you can train away, but if you don't, it can get very dangerous.

ORIGINAL: Amina

Also, I think he wants to show us who is boss. So even if we had a better treat, I think he'd keep holding onto the bone.

I don't think you realize how a dog thinks. They don't have the same ideas as we have. It's not "I'll show these humans..." it's more like "yummmm...I love this bone, I don't want to let go, grrrr..." I can't really read dog's minds but you get the idea. Try a pice of meat or something that smells really good. You will never know till you try how he will react. You just left him confused without teaching him what he really needed to learn. It takes lodes of time, reading and practice to know how to read a dog and train a dog.

What I meant was that I feel that Zok and I are having some power struggles. His growling, whether he has something in his mouth or not, seems to be a sign of dominance (or maybe defensiveness since he was mistreated before). The bottom line is that I would like his growling to stop, I don't want to be worried that he will bite, so that we can have a trusting relationship.

You actually don't want to inhibit his growling - a dog that feels like he can't growl, goes right to the bite.  The way to establish that you are his leader is through obedience training, and not being harsh with your voice.  My advice would be to find a good training center nearby and sign up for classes.  Your trainer will then be able to work you and your dog through this.  It sounds like your dog is running the show at your house LOL.  Some simple steps to change his role:
No couch or bed privileges.
He must work for whatever he gets.  If he wants to go out, ask him to sit first.  If he wants dinner, make him sit/wait till the bowl hits the floor - he doesn't sit/wait, oops dinner goes away for 20 minutes.  Second time, still doesn't sit/wait, oops, dinner goes away till the next mealtime.  (This is, of course, assuming he knows how to sit and wait). It's called NILIF training.  Nothing in life is free.  You never have to get upset or angry, just consistent, using the right techniques.
Both of these sites have trainer searches. 
www.apdt.com
www.clickertraining.com
My advice about the food guarding still stands - it doesn't matter what the cause is, it still needs to be dealt with in a manner that doesn't put you or your family at risk.

Also, I think he wants to show us who is boss.

 
no. Resource guarding and growling have nothing to do with dominance. My alpha bitch is extremely dominant-- but she would NEVER be so rude as to try to take a bone directly out of the mouth of a subordinate dog, and a subordinate dog would probably growl and bite her if she did try such a rude behavior. To a dog, Ownership Trumps Dominance.  Growling is simply a communication-- I'm not happy with what you are doing.
You should pro-actively work on these behaviors. Spend some time every day teaching the dog to drop things on command, in exchange for a treat or toy. Don't start with a lovely prize bone, start with something boring the dog will readily drop. Once you have the behavior well-established, next time the dog gets hold of bone you need him to drop, the odds of him doing so are very high.
Anything you find the dog growls at you for, you should consider helping him to accept it better. Sy he growls when you touch his feet. So touch his shoulder, offer a treat if no growl. Gradually work your way down the leg.