I just went to the place I use to work the other day and asked about a good food to help our dog gain some weight. What they gave me is called Innova EVO. It's high in protein (42%) and has more calories (1929/lb). You can read about it at http://www.naturapet.com/display.php?d=inn-home
You can also read about the other foods they have. Our dog seems to like it but I have only had her on it for about 3 days so I can't tell much yet. Every dog is different, of course, so I wouldn't necessarily compare your skinny dog to the other one. So, a new dog food is worth a shot. If it doesn't seem to change anything you can go back to the old dog food since she otherwise seems healthy. Let me know what you decide and I'll keep you posted on whether it seems to be working for my dog!
You can also buy the food in dry or canned formula.