If the business letter format is where you are going with this, I would suggest that pointing out some of the elements from the book will help substantiate your case. I think using Houston's depiction of how the camps are, and how difficult they are to live in for those who are relegated to do so might help make the case. Additionally, I would suggest that it might be good to point out that the living conditions in such areas have to be considered an affront to what America represents. The initial paragraph of the letter might be seeking to bring the issue to President Roosevelt's attention, while the middle details what Houston sees in the work. The final paragraph might be where there is some level of an impassioned plea to change such a state of affairs. I am not entirely certain that even in the business letter format, you could steer clear of some emotional element given the nature of what the relocation camps entailed. I believe that being able to speak to this in such a format would make for a letter to the President that speaks loudly to the idea of wanting to bring out change and bring forth the idea that there is a condition in World War II America that is begging to be changed.