Last Minute Shopping
/Birger Pearson, Ancient Gnosticism
Jeffery Kripal, The Serpent's Gift: Gnostic Reflections on the Study of Religion
Alastair H.B. Logan, The Gnostics: Identifying an Early Christian Cult
Christopher Markschies, Gnosis: An Introduction
Richard Valantasis, Gnosticism and Other Vanished Christianities
Florian Ebeling, The Secret History of Hermes Trismegistus: Hermeticism from Ancient to Modern Times
Paul Foster (editor), The Writings of the Apostolic Fathers
Fred Lapham, An Introduction to the New Testament Apocrypha
Hans-Josef Klauck, Apocryphal Gospels: An Introduction
Bart Ehrman, Lost Christianities
If you are looking for the actual texts themselves (Nag Hammadi Codices, Tchacos Codex, NT Apocrypha) they are found in these volumes:
Marvin Meyer (editor), The Nag Hammadi Scriptures: The International Edition
J.K. Elliott (editor), The Apocryphal New Testament
Rodolphe Kasser and Gregor Wurst (editors), The Gospel of Judas together with the Letter of Peter to Philip, James, and a Book of Allogenes from Codex Tchacos: Critical Edition
Feel free to leave your own suggestions in the comments.