This is a very short quick read, written for a middle-brow audience. The author addresses an anthropological/sociological question, identity, by applying history and philology. The book is for some will be tiresome leaving you saying: "Tell me something I didn't already know." But he does make some good points.
First, Arab is the label of a culture, not ethnicity in the region. It is so homogenous, because the Arab conquest that spread Islam was replacing a previous empire--Byzantine--which had replaced a previous one--Rome--which had replaced a previous one--Persian--which had replaced a previous one--Greek, and cultural identity was already weak.
Second, nationality is a Western import, as is race.
Third, since the ninth century, sectarian conflict within Islam was rare, and that the current conflict is an innovation under the pressures of Western modernity.