I don't really want to annoy you with this, but I do think it's quite important to recognise the Bible for what it is and what it isn't. The Bible isn't the book of Christianity, its the book of a very specific form of Christianity that has become dominant.
Back in the 2nd through 4th centuries the orthodox and Gnostic Christians were in quite a fiece competition for believers, the orthodox collected all the books that agreed with their theology and Christology, or wrote their own, and put them all together into what today we call the Bible. If it isn't in the Bible it's because it doesn't agree with the orthodox viewpoint completely, and so shouldn't be in their Bible.
The books of Nag Hammadi, of Jeu and the Pistis Sophia reflect Gnostic theology and Christology and so belong next to, but outside of the orthodox canon because they reflect a very different yet linked set of beliefs.