It's important to note that this book was written in 1975 by a Norwegian for a Norwegian, not American, audience. It's still the best book on Norwegian immigration to the US (and a little bit about Canada) out there. It's readable like any history or textbook, and backed with a ton of research. The first few chapters really drag, and one wonders why the author mentioned all those early immigrants without putting them in context with each other (maybe the Norwegian audience at the time would have known their names?). But overall you get a great picture of immigration from Norway to the US, along with the effects that immigration had on Norway. 

If you're choosing between this one and anything by Odd Lovell, choose this one.