Here is the answer I gave to the same individual in an email:
There isn't a lot of eBook material out on gaining weight, mostly because there isn't a high demand for that information. But if you have thyroid disease or a fast metabolism that makes it difficult to gain weight, you need a plan to add calories and muscle mass to your body for protein and fats to build onto.
Here's a short but decent article on how to do that:
You'll likely also see a few articles about gaining weight if you look up a website related to Thyroid disease. But honestly, you might be better off looking at what body builders do to gain muscle (they have a very special diet). And instead of doing really intense body building and cardio, you're going to want to keep your cardio moderate, limited, and increase your caloric intake by at least 500 calories a day, most likely through a protein and fat shake of some kind if your stomach feels full all the time from eating.