A good general introduction to an enormous subject which brings the Gnostic story right up to the present day and Philip K Dick and The Matrix, via Blake, Jung and Kafka. It contains all you need to know to recognise and appreciate how Gnosticism has become an all pervasive part of contemporary culture, and is at the heart of so many things even when so many people have never heard of it; ignorance of it among the educated classes is quite astonishing, be they atheists or believers. Both have very good reason to find out more, otherwise their position is untenable (see my review of the book 'Better Never To Have Been').
The two other excellent and deeper introductions to Christian Gnosticism are 'The Gnostic Gospels' by Elaine Pagels and 'The Gnostics' by Tobias Churton.
Ultimately the one to aim for is the groundbreaking masterwork on the subject from 1958 which is Hans Jonas' 'The Gnostic Religion: the message of the alien God and the beginnings of Christianity'.
But for the influence of Gnosticism on Western culture in general the book to read is 'Passion and Society' by Denis de Rougement, and almost all of Harold Bloom's critical work on the Romantic poets and their successors, and his 'Omens of Millennium: The Gnosis of Angels, Dream and Resurrection'.
Sean Martin has also written another short book about Gnosticism called 'The Cathars'.