In addition to this focus, the book has the best brief overview of Federal Indian policy I've seen anywhere (about 70 pages). Unlike most books on the subject, the book is amply illustrated with photographs and other images, as well as maps, graphs, and abundant sidebars.
I always use one or more texts on Federal Indian law and policy in my American Indian history courses. Students often call these books dry. When I have used O'Brien's book, students have praised the book and said they enjoy reading it.