This year I started teaching my students to consult the references portion of Wikipedia entries to find viable sources for their papers. While I don't let them quote the content of the page, many of the sources in the references section are great.
This book is sort of like a Wikipedia entry in that it is an awesome book to mine for sources elaborating on evil. For that reason it is a great starting point for people who want to explore the subject from a philosophical perspective. Is it seminal? Not necessarily, but it'll steer you in the direction you need in order to find seminal works on the topic, and it'll provide you with a decent overview.
Evil sells. Most of the time, it is a term used to conjure up age-old anxieties, most of which have little to do with evil if you're a rational human being. When I researched the term for a paper on the connotations of evil, I found so many books about cults and countless right-wing conspiracy texts. Evil is a multi-faceted word, and most of those facets are generated by the batshit crazy. This book will help you cut through all of that without wasting heaps of time sorting through the latest diatribe about the evils of rock music, thinly veiled racist manifestos about the evils of urban culture, the satanic underpinnings in Spongebob, and all the other bullshit the morally anal-retentive try to pass off as evil.