This is 'yet another monster collection' with a few outstanding and unusual features that makes it noteworthy over and above the general usefulness to any GM of having even more monsters to hand...
Conceptually, it's interesting because each beastie has been created in response to a piece of art rather than a load of monsters being dreamed up and then artists commissioned to provide the illustrations. It's an approach which resonates, I once invented a monster when looking at a patch of light on the side of the bath... and it was such a good monster that a whole 3-part adventure grew up around it!
Even better is the way in which they are presented. Along with that inspirational illustration and a standard stat block, the descriptive material - background, behaviour, ecology, etc. - is written up in the style of a 'field report' - not only does this work remarkably well as an in character resource giving verisimiltude to each monster, it suggests the existence in your world of people who go around collecting and describing just like real-world naturalists do. Many suggest adventure ideas even as you read through them. Or perhaps you'll be inspired to play a naturalist character, it's a good role for rangers and certain types of mage for a start, or perhaps a cleric of a nature deity.
The creatures themselves are fascinating, ranging from quite vile (the Arm-Taker) to something that's beautiful (the Diamond Owl) or just plain strange (nearly all of them!). Many will work well in games where barriers between planes or dimensions are weak, you can just imagine them having come from somewhere that is a twisted parody of your own world, having slipped through a crack and possibly being as baffled by this reality as your characters are by them.