For TLDR, scroll to the bottom.

On initial inspection of this book, it offers a lot of new and tantelizing content. To a flexible and amenible Dungeon Master/Castle Keeper such as myself, it is exactly that.

However, it is stated in the Systems and Rules (Chapter 1) that it makes reference of the Wizards of the Coast's 3.0 SRD, as opposed to the core rulebooks of 5th Edition. Additionally, the writers make comment to the fact that from previous editions, the Druid's Animal Companion and the Paladin's mount were removed, and claimed to provide a way of returning those mechanisms. However, in the actual description of Advantages (which may as well be called Feats since they function the exact same way, and are even stated as only being gain-able when a player could take an ASI or feat) does not acknowledge this. I quote from the Animal Companion [General] advantage description: "You gain the ability to form loyal bonds with animals, allowing you to gain an animal companion in the same manner as a druid of the appropriate level." And yet, it was acknowledged earlier in the same chapter that this mechanic for druids no longer exists, and no alternative or replacement is provided. Fortunately, I am both familiar with and in possession of the necessary core books from v3.5, in order to better make sense of and rule fairly on what this might look like in a 5e game.

Additionally, it is apparent the majority of focus was spent on the animal familiars for each class, and the dragon rider paths in the final chapters, as the information for animal companions is scant other than general and by-class comparisons between familiars and animal companions (more than a tad redundant, unless a simplified summary in a sidebar were to be used) and the three advantages specifically pertaining to animal companions: Animal Companion [General], Companion Tricks [General] (which also makes references to rules regarding training and tricks which do not exist in 5e), and the Vermin Companion [General] (which is based off a creature-type distinction that no longer in 5e - Animals and Vermin alike are "Beasts").

Finally, as alluded with my comment on the Vermin Companion [General] advantage, the bestiary specifically and the book as a whole make use antiquated creature-types that are no longer used in 5e: Aberrations and Outsiders of older editions were combined into simply Aberrations, in addition to the aforementioned consolidation.

In summary, I appreciate that the publisher, Troll Lord Games, made the effort to convert the concepts herein from their core game rules (C&C) to the D&D 5e system, however the inattention to detail I find cumbersome. I sincerely hope that they take reviews and discussions on this distributor's (DriveThruRPG) website seriously enough to either:

1. Publish an errata of corrections that would be free to those who've purchased the PDF here
2. -or, better still- Publish an updated version of the book with the corrections included.

TLDR: All in all, the artwork and central concepts are too beautiful to call this work a waste of time and effort. The familiar-themed archetypes for each class, as well as the dragon-rider archetype per class is truly an intriguing and exciting concept. However, the cumbersome nature of making the included options viable in a 5e game should give DM's pause before purchasing, or implementing, this book. I myself am comfortable undertaking the task because of my deep love of creativity, and commitment to Capt Barbossa's view of the Pirate Code: "They're more like guidelines than actual rules." However, this material is hardly plug-and-play quality. Groundwork on the part of the DM will be mandatory.

If it was mentioned, I failed to notice the bit about printing the pages on card stock, and printed it instead on plain paper. While it didn't take much abuse to destroy the tower once complete (I have a toddler, after all) it performed better than I'd expected. I plan to reprint this (with card stock!) and try it again - let the dice shenanigans resume!