These are the books that blew my socks off when I read them. This isn't to say that these are the best books out there, but these are the one that were even better than I expected, to the point where I think they deserve more than 5 out of 5 stars:
The Bestiary: Predators, by Betabunny Publishing. Weighing in at 250 pages, this PDF not only gives stats for dozens upon dozens of real animals, but also covers things like their habitats, eating habits, value alive and dead, whether or not they're edible, life cycle, paw prints, and so much more. It rightfully notes that characters in a fantasy realm wouldn't know as much about normal animals as we do now, and includes excerpts from the historical Bestiary by Pliny the Elder. This book goes far, far beyond the extra mile in what it presents.
Kitsunemori, by Dog Soul Publishing. This Folkloric campaign setting is meant to be fantasy Japan, and it does a spectacular job. Rather than overloading us with new feats, spells, prestige classes, etc., it only has a few of those. Instead, it focuses on interesting new sub-systems like Hearth Magic (used via skills), Geomancy, the Tao, magatama (elemental stones), and more. Best of all, the kitsune (fox spirits) are given so much coverage that it's incredible. Not just new racial information, they have their own classes, and fox magic. This is the best fantasy Japan I've ever seen, hands down.
The Book of Unremitting Horror, by Pelgrane Press. This is so much more than just a monster book. Meant for
d20 Modern, this is a book of nightmares. Each of the monsters is gruesomely drawn, and provides vivid opening fiction, as well as notes of what's left of the victims. The grisly new artifacts and the adventure at the end of the book just help showcase the horror better. This is one of the few RPG books that actually scared me as I read it.
Octavirate Presents Vol. #5: Collectabeast!, by Octavirate Games. Sometimes the best changes are the smallest. Collectabeast thematically brings Pokemon and Digimon to d20, and does so masterfully. The new templates make a series of small changes that radically alter existing d20 monsters to adorable, nonlethal variants, adding new depth to familiar creatures! The new artwork and names given really spur the imagination, and with their wide variety of optional d20 rules and alternate genre support sidebars, the possibilities of what's given here are mind-boggling. This is easily my most favorite product from what's rapidly becoming one of my favorite d20 publishers.
