This book has been an eye-opening purchase for my C&C games, and the best part is that it's easily portable to any 3e D&D or even old-school games like AD&D, 2nd edition, OSRIC, Swords & Wizardry, and others.What I love is that every class gets several options that make sense, and aren't overpowered, as there are costs associated with gaining, powering up, and maintaining a familiar, animal companion, allied spirit, or whatever. Brilliantly, the writers came up with several alternative costs for these traits, and they mirror previously available spells (summon familiar, primarily), so balance is maintained and you can rely on different resources, everything from gold/treasure to XP to "Feat-like" Advantages that every class can access. The beauty of these options is that they are both simple *and* they mirror the types of resources you might find in any D&D or d20-based game, so slotting in these additional abilities is a breeze.The level of detail is also fantastic, and not overly verbose in its presentation. There are the "general rules" for familiars and animal companions up front, and then each class gets its own chapter with more specific details. This way, you have a general listing of familiars and animal companions to choose from, and the special abilities they provide, but then you get exceptions and more "thematic" lists by class, so that each choice truly feels like it is a fundamental part of your chosen character class, rather than just having a single list for all classes to choose from, leading to min-maxing or (possibly worse) bland, "samey" choices.A couple appendices round out the book, expanding on the nature and role of familiars in the game by describing new spells, magic items, and monster stats directly related to the ideas presented earlier. You could totally have a world where every character has a familiar/animal companion that doesn't break the game, or simply make the few, rare characters that do have familiars feel like the familiar/companion is an integral part of how that character works in the game.My 1-star ding comes from the *very* minor quibble of organization. Presenting some of the rules up-front but not in the individual character sections causes a lot of page flipping to grok the rules and choices presented for each class. Furthermore, forcing some of the class-specific tables of familiar choices to appear in the class sections was slightly jarring, as these tables are similar (but not the same!) to the general ones in the earlier chapters, so it felt weird without presenting better headings or organizational tools to understand this. It's a very minor gripe, probably more due to how my brain works rather than a flaw in the book's organization.In summary, this is a useful addition to my game, and its versatility allows it to be easily integrated into any D&D edition or d20-based RPG. Check it out!