I find this article on Gnosticism from the Jewish Encyclopedia a creditable source for those of you looking for an overview.
I have put Elaine Pagels' The Gnostic Gospels on course reserve, as an example of a true-blue conspiritorialist's take.
However, for a proper scholarly treatment, we have Rethinking ""Gnosticism"" : An Argument for Dismantling a Dubious Category by Michael Williams available from our eBrary. If you are truly interested, I strongly recommend you begin here.
To see a gnostic in action, read Thomas Vaughan's COELUM TERRAE Or The Magician's Heavenly Chaos here online. Thomas, brother of the poet Henry vaughan, wrote proxily such works as Anthroposophia Theomagica and Anima Magica Abscondita (the titles of which tell you very much about the gnostic sensibilty) under the pen-name "Eugenius Philalethes." Dr. Alan Rudrum, SFU Emeritus Professor, is the leading authority on Thomas Vaughan, and edited his collected works for Oxford UP. See his "Thomas Vaughan” in The Dictionary of Gnosis and Western Esotericism, to be published by Brill, Leiden, The Netherlands (in press).