If you've never heard of the Nag Hammadi documents, think Dead Sea Scrolls but less well known. The Nag Hammadi Scriptures contain English-language translations of fourth-century papyrus gospels unearthed in Egypt in 1945. I relied on an earlier compilation of the translations in my research for The Awakening series.
Inside, you'll find many lost texts the official Catholic Church rejected, including the Gospel of Mary, the Apocalypse of Adam (where Adam speaks to his son Seth), the Gospel of Judas, the Gospel of Thomas, and two I used in the Awakening series, The Trimorphic Protennoia and The Thunder, Perfect Mind. (You'll see reference to the latter in Book 3, The Conflagration, which will be released May 17, 2017.) Both of those texts are poetic and both lend themselves to a focus on the feminine aspects of God. I liked reading them because they interested me even if I hadn't been looking for inspiration for my fiction.
Other texts I found hard to follow, and still others seemed so out of whack (that's a technical term) to my modern-day eyes that I wasn't surprised the Church disregarded them. Whether they made sense at the time and were rejected as contrary to official Church teachings, I don't know.
This edition in particular of the Nag Hammadi Scriptures makes me cheer for Amazon and other platforms for selling books on line. When I first learned about these manuscripts, the compilation of translated versions was only available through academic publishers at a cost of over $700. Later, I bought the edition I still have through Amazon for about $60. As I write this, the price is $12.99 for Kindle and $15.48 for paperback, with many used copies available for less.
If you are interested in the origins of Christianity and how and why some of the earliest texts were excluded from it, this is well worth the read.