This book’s brevity is part of its appeal yet ultimately what makes it a frustrating read. While Irenaus wrote in Greek, it’s doubtful that this book is from the Greek. As far as we can tell, it is an English translation of an Armenian translation of a probable Greek text.
It does a nice retelling the story of salvation. Unlike later Fathers, Irenaeus keeps the Gospel narrative of what God-has-done-in-Christ-for-us in the foreground. Metaphysical speculation is kept at a distance.
Further, it lacks the anti-Gnostic polemic that you find in Against Heresies. I’m all for bashing and trashing Gnosticism, but you have to admit that made for a very painful reading. This book is wonderfully more focused.
The “book,” such that it is, really isn’t that deep or profound. It is more a running commentary on the main story of the Bible. That’s a good thing. That is something that would be slowly eclipsed over the centuries. On the flip side, though, he isn’t telling you anything you don’t already know.