Many stories and fables place a “familiar spirit” at the side of a wizard, witch, or other practitioner of the “dark arts”. These familiar spirits are often some sort of demonic force that has taken on a corporeal form, usually that of a small animal. The image of a black cat at a witch's side has thus become an iconic one, and thus it's not surprising that spellcasters in fantasy roleplaying games should have the opportunity to have one of these familiar spirits as well. In Pathfinder games, familiars are generally more benevolent than their counterparts in the stories and aren't generally demons in disguise (though the more fiendish kinds of familiars can also exist). Nevertheless, familiars fulfil a similar role: they aid their masters in various tasks and, in the case of the witch class, are the source of their magical powers.
Despite the fact that familiars have been part of the game since the days of the find familiar spell in 1st and 2nd Edition D&D, they have gained a bit of a reputation for being a bit...well...useless. While not a reputation I fully agree with, familiars are relatively weak and are of extremely limited help in combat, where they will die very easily if not carefully protected. Of course, not everything should be strong in combat, but even outside of combat, familiars provide only limited benefits to their masters. Familiars can be great for roleplaying as characters to interact with (and I have seen many very fun familiars in my own games), but beyond that, they are very limited in what they can do. Ever since Pathfinder introduced options for wizards and sorcerers to not have familiars, I've found—in my own games, at any rate—that most players have gone for the alternatives, such as the bonded object for wizards.
I think some of the issue may come from the fact that there have been few options for modifying familiars in the way that so many other things in the game can be modified. The Improved Familiar feat exists along with a smattering of other feats and spells that affect familiars, but for the most part official sources haven't really done much with familiars (although third-party publishers have occasionally tackled familiars). Beyond a minor skill bonus variance based on the type of animal chosen for familiar, every familiar is pretty much the same. Even a witch's familiar, which is an integral part of the class doesn't offer much in the way of new abilities.
Animal Archive began to rectify this situation by introducing archetypes for familiars, along with new feats, new kinds of familiars, and various other ways to modify characters' familiars. But Animal Archive covers animals in general, not just familiars, so there is limited space in that book to greatly expand the options for familiars. Familiar Folio is the first official Pathfinder book dedicated entirely to familiars, and takes the needed step to expand familiar options considerably with new archetypes, feats, spells, magic items, and more.
Familiar Folio is filled almost entirely with character options. There is very little background campaign information apart from a brief section at the beginning which looks at the kinds of familiars preferred by various groups from across Golarion, such as the Arclords of Nex, the Aspis Consortium, and the Hellknights. Here and there throughout the book, there are occasional brief mentions of Golarion groups or places, but for the most part, this book isn't really all that tied to the Pathfinder campaign setting, making it easily usable with any campaign setting without any adaptation. The focus here is entirely on mechanical options.
The book's introduction contains the aforementioned favoured familiars as well as information on the role of familiars in the game, giving players who aren't entirely sure what to do with their familiars some basic ideas. A sidebar provides specific tactics for using familiars in combat in ways that won't get them killed, such as providing them with magic items like wands that they can use via their master's Use Magic Device skill.
After the introduction, Familiar Folio provides several pages of archetypes, first for character classes that don't normally gain familiars (like fighters and paladins), then for those classes that do (like wizard and witch), and finally for the familiars themselves. At six pages total, there are a lot of options here, many of which are quite interesting. They provide the variation for familiars that has been missing from the game. The pact wizard archetype, for example, brings in the type of familiar that comes from a bargain between the wizard and an extra-planar being. The homunculist, as its name implies, allows an alchemist to gain a homunculus as a familiar. The archetypes for classes that don't normally gain familiars allow players to explore other kinds of stories where there is a bond between a person and an animal. The chosen one archetype for paladins makes the familiar an emissary of a divine force that has chosen the paladin for some kind of great fate (there is a corresponding emissary archetype for familiars that goes along with this paladin archetype). The familiar is there to guide the paladin along the correct path (the archetype essentially models a common manga/anime trope seen in things like Sailor Moon).
However, I'd say that the archetypes for the familiars themselves are the most interesting for the simple fact that there have been so few such archetypes in the game previously, whereas character classes have scores of archetypes to choose from already. The decoy archetype allows the familiar to eventually mimic its master's voice and even assume its master's form. For those who want their familiars to be more useful in combat, there is the mauler archetype, which gains increased strength and can grow into a larger size. Overall, there are some great archetypes in this book.
After the familiar archetypes, Familiar Folio moves briefly away from archetypes (though it soon goes back to them) to look at how to determine the statistics for familiars of an animal type not covered in any existing books. It offers two systems: the simple method, which is simply to use the stats of a similar animal; and the advanced method, which involves starting with a similar animal and then adjusting the stats as necessary to make them fit the new animal more closely. There is also a table listing numerous animals that already have stats and suggested other animals that are similar to them. Both methods are quite straight-forward and easy use. They may seem a bit obvious for experienced players, but newer players and gamemasters will likely find them very useful. The only criticism I have of this section is its odd placement in the book, coming between two sections on archetypes. It would have made more sense later in the book, either before or after the sections introducing new familiars. This is a very minor issue, however.
The next section introduces school familiars. This option allows specialist wizards to attune their familiars more closely to their school of specialisation. A character first takes the School Familiar feat, which then applies the school familiar archetype to the character's familiar. This is quite a significant increase in a familiar's power as it allows the familiar to use the granted abilities of its master's arcane school (although expends two uses from its master's total each time it uses one of these abilities). It also grants the familiar a new ability based on the arcane school. There is also a Greater School Familiar feat, which grants the familiar a further power based on the master's arcane school.
The next section contains additional options for familiars: bloodline familiars and patron familiars. Although the book does not call these archetypes, they are essentially very simple archetypes, in that they swap out or modify a single ability for another. Bloodline familiars give sorcerers of any bloodline the option to gain a familiar by giving up their 1st-level bloodline power and getting their bonus bloodline spells one level later. Each bloodline provides the familiar with one specific new ability. Since there are a lot of bloodlines out there, this section can only provide a sample selection of bloodline familiars, but there is enough variety here to give GMs a starting point for creating bloodline familiar abilities for unlisted bloodlines. Patron familiars allow witches' familiars to gain an additional ability tied to the witch's patron in return for delaying access to bonus spells by one level. Like the sorcerer bloodlines, not every patron is listed here, but there are enough for GMs to extrapolate abilities for the missing patrons. Overall, this section adds a huge variety to familiars.
The next few sections contain new feats, magic items, and spells. The feats include a new category of feats called familiar feats. These feats are taken by a character with a familiar, but they directly affect the familiar instead of the character. It's a nice little way around the fact that familiars can't generally learn new feats beyond their starting feats, giving them new feats at the expense of their masters' feats.
The final few sections of the book introduce the stats for various new familiars, including improved familiars. These include things like chickens, kakapos, peacocks, and penguins. They're not all birds, though, as there are also koalas, pufferfish, wallabies, and more. The last of these sections includes “unusual” familiars. These are not animals, but instead other kinds of creatures (there is a construct, a vermin, and a plant). Yet they are also not improved familiars, and as such, can be taken by any character who can gain a familiar. They're a great way to expand the definition of just what a familiar is and they definitely live up to the term unusual (especially the ioun wyrd, which is a construct made from gemstones, lodestones, and bits of granite).
These days, I generally find myself growing tired of books that are almost entirely crunch. There's so much out there already that, unless the new stuff is exemplary, it just ends up seeming forgettable. Familiar Folio, however, is a definite exception. It expands the game in an area that has seen very little expansion, and thus is far more memorable. It is a very welcome resource and will add needed new levels of fun to familiars.