Unfortunately, much of the recent interest in Gnosticism is either opportunistic (such as the attempts by feminists to interpret the Sophia character in terms of feminist liberation ideology), manipulative (such as the usurpation of its ideas as myths by Carl Jung, who at one time owned the Nag Hammadi manuscripts, and viewed its ideas through the distorting lens of his pseudopsychological theories), or superficial (which has lead to an association in many minds of Gnosticism with New Age intellectual "mushology"). Thus, books on this subject vary widely in their usefulness, scholarliness, and accuracy.
Gnosticism: New Light on the Ancient Tradition of Inner Knowing
by Stephan A. Hoeller
In no sense is this a work of mushology, but in many places the author doesn't make clear whether he is describing something the Gnostics believed, something the Church said about them, something that modern-day scholars say about them, or something that he himself believes. This makes the book difficult to use as a source of information. Hoeller also briefly describes the history of adherents to Gnosticism since the fourth century. Gnosticism is portrayed in such a positive light that one sometimes suspects that Hoeller himself might be an acolyte.
Gnosis: The Nature and History of Gnosticism by Kurt Rudolph.
A more scholarly work, translated from German. Printed on cheap paper. Although this book paints a clear and accurate picture of the Gnostics, the complexity of their beliefs, and their relationship to other sects, the translator's writing style sometimes is a bit dull. The author tends to dwell on the less interesting aspects of Gnosticism.
The Nag Hammadi Library: The Definitive Translation of the
Gnostic Scriptures Complete in One Volume James M. Robinson, editor.
Printed on very cheap paper. Excellent compilation of translations of the original manuscripts. Where the original text was indecipherable, it is indicated.
The Gnostic Bible: Gnostic Texts of Mystical Wisdom from the
Ancient and Medieval Worlds--Pagan, Jewish, Christian, Mandaean,
Manichaean, Islamic, and Cathar Barnstone and Meyer, eds.
Not as complete as The Nag Hammadi Library but includes material from other sources, as the title says.