Liquids like Ensure are exactly what I'm talking about, taken by mouth or through a feeding tube, rather than intravenous nutrition (TPN). Other brands people use include Modulen IBD, Vivonex, Peptamen, E028, etc. Some people take them as their primary treatment; others use them as you have, strictly for weight gain, or for growth in kids (and those, too, are important uses that I am covering in the book, and would love to have people's input on).
Ensure can work really well, by the way -- there have even been a few studies of its use to induce remission in Crohn's, but they were published back in the 1980s and few people are aware of them. I want all that information out there where people can find it, rather than having to dig it up in a medical library themselves!