Reddit reviews Vikings: Warriors, Raiders, and Masters of the Sea (Oxford People)

Here's a link to the book on Amazon. It's pretty basic, but fairly well-researched and a good introduction to Viking history. It has some neat little bits here and there I was unaware of and is a nice little coffee table book or bathroom reader.

The book describes the Western half of Denmark as being a "desolate sandy heath", so it seems the land is simply geographically unsuitable for farming based on the composition of its soil, something I imagine was the case ever since the glaciers of the Ice Age receded.