This book was written, I believe, to be a general introduction to Gnosticism for people with no prior understanding. It is very good in that it does give a brief overview of the major concepts that Gnosticism entails as well as similarly brief overviews of the major Gnostic sects that have appeared through out history.
On the negative side, the book is brief, after all it is only an introduction, and will need to be supplemented by other reading before a modern reader can get a true idea of what Gnosticism was in history, and is today. I would reccomend supplementary readings of "The Allure of Gnosticism" Ed Robert Segal, for an overview of Gnosticism from a genererally psychoanalytic, Jungian perspective, and Elaine Pagels "The Gnostic Gospels" for a scholarly, but very readable treatment of the ancient gnostic literature. I would genuinely suggest a modern beginner should read this supplementary work before launching into a reading of the Nag Hammadi Books because they are confusing and off-putting until you can approach them with a good idea of the "Inner language" or jargon of the times.