I would suggest you either start with Introduction to Gnosis or Fundamentals of Gnostic Education. The Gnostic Prayer Book is excellent for learning prayers of protection and other practices to apply to your daily life. The Gnostic Bible is a work replete and heavy with Kabbalistic knowledge, which will take a lifetime of study and practice to comprehend and realize. It is better to study Tarot and Kabbalah by Samael Aun Weor before even approaching The Pistis Sophia, since without knowledge of Kabbalah in one's heart, it is like attempting to read Shakespeare without knowing English.