I would give this book a far higher rating. It is to this day the "go-to" book on this subject. Not only is it packed with information, covering a multitude of subjects, it is written in a thoroughly readable style, with authority and wit.
Another reviewer's statement that the author relied on "merely a handful of sources", and mainly on the Sefer Hasidim, is utterly absurd. While the Sefer Hasidim is deservedly well-represented, a glance at any page of notes will refute that reviewer's ill-considered statement. Trachtenberg had deep knowledge of the sources, from responsa to minhagim to talmud and many, many other obscure (to the average reader) sources, both in print and manuscript. His source list covers 15 full pages (5 pages of Hebrew sources + additional literature)!
Trachtenberg focused on the medieval European world that sprang from the Hasidei Ashkenaz, the school of Rabbi Judah the Pious that produced the Sefer Hasidim, thus its prominence (but very far from exclusivity) as a source. He chose this area because of its richness, and its absorption and "Judaization" of the folk beliefs current in the local cultures within which the Jews existed. His sources, within his area of focus, are indeed extensive, and more than sufficient for an introductory book of this nature, which aims to give an overview of a complex phenomenon.
On a personal note, I have collected and translated Hebrew magic manuscripts (kabbalah ma'asit) for many years, counting more than a hundred manuscripts and rare books in my modest collection. Trachtenberg's book provided the inspiration for choosing this area of study; it is still the best introduction by far for the non-specialist.
Read this book with confidence, and mostly, with pleasure.