As a good reference book, this one is hard to beat. Waagenaar personally interviewed people whom Mata Hari knew as a child and as a young woman. The best (and most interesting) was her maid and confidant, Anna. It didn't hurt that Waagenaar (who had a notable career behind the camera in Hollywood) was, himself, Dutch and could speak first hand and knowledgeably with people who were only a generation older than himself. There's not as much speculation here as other works.