Here are some high-calorie solutions for skinny folks looking to pack on muscle. All of these are whole foods, most of them are pretty "clean" - no trans fats, little or no processing, etc. Note this isn't "low fat" or "low carb" and isn't intended to be. If you want to gain muscle, you have to take in a lot of calories. Here are some (easy) ways to do it.
Disclaimer - I eat most of these foods. I often eat a fair amount of them. I don't have a nice track record of sudden gains to show you; it took a long time and a lot of eating and lifting to get where I am. Finally, I spend at least 1/4 of any given year dieting down to compete, and probably another 1/4 to 1/2 eating at maintenance so I can stay within an easy cut of my weight class. But I love food, and I love a good excuse to pack in a lot of it without regretting a lot of processed crap the next day.
Dried Fruit: Dried fruit is a fat-loss no-no. Dehydrated fruit leaves all the sugars but reduces the overall volume of food. You could eat maybe 1-2 peaches before you feel like you've eaten enough, but you can knock off a dozen dried peaches without really filling up. A fat loss no-no equals a weight gain goldmine.
For example, a cup of raisins - pretty much two handfuls - is almost 115g of carbohydrates and over 400 kcals.
Dried fruit can be a bit too sweet by itself, though, so you can add some nuts.
Nuts: Mostly fat, these are extremely calorie-dense foods. Low volume, high calorie, easy to eat = easy way to get extra calories.
For example, a cup of almonds is 820 kcals, and has 30g of protein and almost 17g of fiber. Peanuts (not actually a nut, but still . . .) are almost 875 kcals for the same volume, with 40g of protein and almost 14g of fiber.
A cup is a lot of nuts for one sitting, but not over a day. Measure it out and add it in.
If you're trying to put on weight but can't seem to get in enough food, take a look at those two. Calorie-dense, easy to eat, portable, and great together or separately.