This book immediately took me back to my childhood obsession with bestiaries -- the Monster Manual I found in the public library when I was 8, the Dragonology series, various "encyclopedias of fairytale", and so on. I love monsters, and I love Eberron, and I love how the book places these monsters in the world of Eberron, creating a living, breathing, magical ecology.
I especially appreciated the monster variants; the idea of different kinds of allips or ankhegs helps further the idea of Eberron as this is a world that reacts like the real world. The idea that monsters would evolve different biomagical variants based on the needs of their environments is a parallel to the idea that humanoids would develop magic to ease daily life, and for more conceptual monsters, the idea of monsters born of different, varying concepts helps make the world feel fuller and richer. I was also taken with your ideas on the armanite -- that it's specifically a centaur's idea of a fiend.
The one thing that detracts from the text is that it doesn't tell DMs where to find some of these monsters -- I have the Monster Manual and Volo's, and I recognize some of the names, but I don't know where to find aldani or archons of redemption. Cross-referencing would make this book easier to use.
That said, the clear love of Eberron, and of monsters, shines in this work, and the opening fiction supports the concepts wonderfully. I very much enjoy Fortunine and Immok, and Ardiane and Rhen have fun banter. I'm excited for the coming volumes!