The social norm these days is for everyone to be somewhat overweight and thus people regard a healthy weight as looking too skinny. If you have a light frame or you are an althete the BMI guidelines may not be accurate for you.
Assuming you are eating when you are hungry and you are not experiencing any medical issues such vulnerability to infection or loss of menstration you are probably not at risk as your BMI is only mildly underweight.
If you are concerned you can get detailed body composition and bone density tests which are more accurate than BMI.
Adding healthy fats to your diet is a good idea as you want to make sure you are getting enough omega-3s.
Its overall calories that count, you dont even need to focus on the calorie dense foods such as nuts, seeds or protein rich vegan foods such as tofu etc. Although these are obviously easier to put on weight as there calorie dense.
If you are having trouble maintaining your weight then a online calorie counter/tracker such as fitday.com can help see how much calories your getting. From there you can up your food intake.