My opinion - this is cool! Though players may find its in-game use sort of limited, its contents can add an entirely new and unexplored dimension to game play. Some of the books are described as being valuable only as a collectible, but others have potential to be something more. The in-game use of the books is also described by the authors in an open-ended fashion - i.e. "Though this book could be useful for someone with the right skills, for most it just helps their chances at being able to do something basic". But any decent GM/DM won't have too much trouble in planning out side-quests and adventures around any of the books that this product describes. I mean, who doesn't want to collect various bits and pieces from monster corpses (scales, feathers, organs) and make some sort of cool potion out of them!
[5 of 5 Stars!]