A conclusion (Codex III, p. 68) describes how Seth himself wrote the Gospel, taking 130 years over the composition – a rather surprising amount of labour for a work which can scarcely be described as a literary masterpiece.
A collection of Christian fanfiction (mostly) in the first two centuries following his death.
The real strength of this collection is that you aren't thrown to the wolves after the general preface. Each piece has accompanying introduction, providing essential context as to what is being written and why (a few clashes with competing Jewish communities come up). I really appreciated that.
As for the material itself, there is a good, if unintentional, comparison with the canonical Gospels via Marcion's Gospel, a substantially reduced version of Luke's Gospel. I personally found the canonical material, whatever its faults, superior to the other material, suggesting the early church got it "right" when composing the New Testament (from a very ignorant lay person's perspective). However, the five stars is for the accompanying commentary - I cannot practically give stars to historical material that had purposes other than for your enjoyment as a reader.
The book is a useful insight to the vibrant exchange of views around the early precepts of the Christian faith. Pair it with a general history of the very early Christian Church and I think it will really deepen your understanding of Gnosticism and other competing versions of Christianity. It also provides background text for certain artwork, such as Mary with scarlet, or Jesus giving life to clay birds.