This text is one of the first (if not the first) studies of the phenomenon of the double as it appears in western literature and culture. It purports to be grounded in the principles of classical psychoanalysis, but is really rather disappointing on that score, and others. If you are looking for a text that catalogs 19th and early 20th century appearances of the doppelgänger in literature, this text will fit your needs. Rank describes these appearances fairly exhaustively, and he separates the examples of the doppelgänger into helpful categories. However, he touches only lightly on the psychoanalytical significance of these examples and he refrains from attempting any general theory of the doppelgänger. The final page comes close to a conclusion, but there is a certain level of ambiguity throughout which is, perhaps, justified given that he was breaking new ground.
It is also important to remember that the book is dated and therefore limited in scope. For example, Rank spends time discussing the mental trials and traumas of the authors who use the motif, ascribing an autobiographical basis for its employment. This is potentially the case in some of his examples but is not relevant in contemporary terms. Rank makes some general points about how and why the double appears, i.e. the paradoxical case of the suicide who destroys himself, projecting his fear onto the double, in order to escape the fear of death. Chiefly he defers to Freud, leaving to him the finer points of psychoanalytical theory which may be said to underlie the appearance of the double. (See for example Freud's essays 'The Uncanny', 'Beyond the Pleasure Principle', etc.) Rank provides a chapter on the anthropological significance of the doppelgänger, but the chapter disintegrates into a further survey of the literary appearances of the motif, which had already been covered ad infinitum in the previous chapter.
There are a handful of contemporary studies of the doppelgänger which are far more in depth, although they are also heavily theoretical (see Vardoulakis, Webber). Rank's study is brief, a mere 86 pages. It is certainly helpful and worth the read, but it remains only a preliminary study of this phenomenon.