Nearly without exception, all handbooks of new religions contain a chapter on Scientology, among them:
Eileen Barker, Professor at the London School of Economics: New Religious Movements: A Perspective for Understanding Society (1982), and New Religious Movements: A Practical Introduction (1989);
Bryan Wilson, Professor Emeritus at Oxford University: The Social Dimensions of Sectarianism (1990); and
J. Gordon Melton: The Encyclopedia of American Religions (1993).
It is also categorized as a religion in most German publications such as Lexikon Der Sekten, Sondergruppen Und Weltanschauungen (1991).
As a good example, Bryan Wilson writes extensively in his book about what makes Scientology a religion.