You can find virtually every historical Gnostic text at the Gnostic Society Library, including several translations of the texts of the Nag Hammadi Library, the texts of the Church Fathers who spoke against Gnosticism such as Iraneaus, and many of the works of early Gnostic scholar G.R.S. Mead. There's also a very good section about Valentinus and the Valentinian school of Gnosticism, the largest and most influential Gnostic school.

As for books, Marvin Meyer and Elaine Pagels are the most prolific authors as far as popular history books. Meyer's "The Nag Hammadi Scriptures" is a good resource, containing translations of the entire Nag Hammadi Library (and the Berlin Codex), with footnotes and introductions for every work within the library. I have yet to read other books besides that, however, so I can't really comment on any of the others, but hopefully that's enough to get you started.