First, this is an excellent book! It is an academic book, and as such, it may not be for everyone. Kathryn Gin Lum writes well. Her prose is concise. The academic aspect can be found in the details of the history. I am using aspects of her book in the religion course that I teach.
Gin Lum shows how American Protestantism categorized non-Protestants, even including Catholics at certain points in early America, as heathens. However, heathen was primarily used to refer to Native Americans, African Americans, Asian immigrants, and non-Christians to whom American Protestant missionaries had been sent. This would include native Hawaiians.
The category, heathen, served many purposes. For example, the "heathen" were supposedly unable to properly care for their land and make it productive; therefore, rendering heathen land available for conquest and occupation by a more industrious Christian population. Heathens were also bad parents, allowing Christians to take children away to properly educate them. Heathens could not truly care for themselves; they needed a white savior to direct them towards civilization.
The term, heathen, easily elides into race, as the heathen were typically non-white people. The inadequate qualities of the heathen were (and are) applied racially.
I gave the book 4/5 stars, because I believe that her argument would have been vastly improved with one in-depth chapter on secular literature that demonstrated how race and heathen were interlinked. She concentrates on Protestantism, while also claiming that her results are larger than Protestantism. I think that she is correct, but it could be better demonstrated.