The ARG builds upon concepts originally introduced in Advanced Player's Guide (APG), and I would say that Paizo largely expects you to own and use the options in the APG if you want to use the ARG. The ARG uses alternate racial abilities, alternate racial favored class abilities, and archetypes, all introduced in the APG first, and the archetypes available include archetypes for the APG base classes as well as the base classes introduced in Ultimate Magic and Ultimate Combat. I take this as a blessing and a curse, a sign of "supplement bloat" to a degree. If you own and use all the options in the APG, then you'll have no problem further supplementing your games with the ARG. If you don't, the book may be largely useless to you. My own personal dilemma comes from the fact that I do not use any supplementary base classes in my home games, so all the archetypes that reference the APG, UM, or UC classes are useless to me and I feel like I have less content available to me. I realize there is a rock and a hard place situation here--if no prior supplementary material is referenced, then those who do use those materials also feel short changed. I will give Paizo credit that they explain what all the new concepts are so that if you don't own the APG you will still understand how the various alternate abilities and archetypes work, but this book, more than any other supplement to date, has rung to me as "you must have collected all four to be able to use this book properly." This situation is very much YMMV, and I point it out simply so that others may be aware.
The book is divided into four sections, the first three of which are basically variations on the same theme. Every race depicted in these three sections gets new alternate racial abilities, favored class abilities, archetypes, as well as race specific feats and equipment. There's also extensive flavor text for all races, and lovely art to accompany it.
The first section looks in depth at the core races, dwarf, elf, gnome, halfling, half-elf, half-orc, and human. It expands upon their description text from the core rulebook, and it adds more alternate racial abilities and favored class options than what are listed in the APG, although what is listed in the APG is also repeated. Every core race now has a unique favored class option for every core and base class available in the game. Core races also get three new archetypes per race. I appreciated the extra detail in this section, although I did feel to a point like I was re-reading stuff I had already in the core rulebook or APG, but the new material is also valuable. I especially enjoyed most of the new feats and equipment.
The second section basically does the same thing for non-core but popular races for both PCs and NPCs, such as familiar "savage" humanoids like goblins, kobolds, and orcs, as well as the various races oldschoolers would collectively refer to as "planetouched," like tieflings and aasimar. This section goes into a little less detail--although what is there is still comprehensive, and each race has only two archetypes and less new material. Likewise the third section repeats the formula for very uncommon races, including a number of "animal-folk" like catfolk and tengu, as well as some unusual planar influenced races like the suli (jann-descended) and the wayang (shadow-descended). My understanding is a lot of these races originally appeared in Pathfinder setting material (I don't usually buy from the Companion or Chronicles lines so I can't speak to how much new is introduced). These uncommon races get only one archetype and again less information in general, but are still presented with well written descriptions. I did feel short changed on the amount of abilities--in particular, I think at least the races in the second section could have used more racial abilities and favored class options, to match the first chapter. I would have been willing to sacrifice some archetypes from the first or second chapters for the extra space. In fairness, for racial options, I generally prefer the alternate abilities to race-specific archetypes, but that's in part because I grew tired of racial class restrictions as far back as the 80s, and it's a concept I have no desire to see return to the descendants of AD&D.
The final section is the race builder, a system to allow GMs to build brand new races from scratch. It uses a point based system to build races, with "standard," "advanced," and "monstrous" races as categories for how many points you should use to build a certain race and how many abilities they may be able to have. The system includes a strong caveat that the race building rules are guidelines, and that the entire section is to be used at the GM's will and with the GM's discretion, which I do think must be borne strongly in mind by any and all users. I participated in the ARG playtest and review and I think the designers did take some of the most important feedback to heart -- for example, that not all core races needed to be shoehorned into a 10 point build, when obviously some core race abilities were truly more or less valuable than what the developers originally tried to assign them to be to make them fit a mold that they'd never been put into in the first place. This makes some of the point costs and assignments more sensible than they were than in the playtest, and at least I am fine with the fact that some core races come out to more or less than 10 points. From what I recall from the playtest, I think few will protest.
Still, I wish more player feedback from the playtest had been taken into consideration for the final product. My particular, though minor, peeve is that there are too many too-specific abilities -- a racial ability that grants you the ability to work with stone, but no such thing for working, say, with metal or clay or leather. Of course you can substitute in such things yourself, but I would have preferred many of the choices to have been made more generic to begin with, rather than force us to wing it in a system that already presumes a fair amount of "winging" to start with.
Nonetheless, it is a solid system that will give race tinkerers a lot of content to work with--again as long as all is taken as firm guidelines than laser-etched rules. I also like that the section provides some advice for how to deal with races of different power levels. Since Pathfinder did away with the problematic "level adjustment" concept from 3.5, the race builder rules offer different alternatives for having very racially "mixed" adventuring parties. The basic rule of thumb is generally to take a powerful race and remove abilities so that they match core more closely, or alternately to use the race builder to add abilities to weaker races so they are better balanced with stronger ones. Some may not like the idea as much as I do, but I appreciate firmly getting away from the character level issue entirely.
Production-wise, the book matches the high standard of quality that other PFRPG hardcover books meet. It is a good length, printed cleanly and clearly on glossy pages with beautiful artwork that enhances but does not distract from the text. The spine is solid enough, and I ordered mine from Paizo directly, whose dutiful golems placed corner protectors all over the book so there was no chance of it getting battered in shipping. My one layout nitpick is that for each race listed, the standard racial abilities are listed in a separate box at the bottom of the page. The way the pages are designed, it is very easy to read the race's description and then go straight into alternate racial abilities before you've managed to read the standard racial ones first, and makes it hard to cross reference between the two.
The ARG is a very nice supplement, with a well-organized and vast amount of information on Pathfinder character races. While I wouldn't consider it required reading, if character racial options are what's up your alley, then it's THE go-to sourcebook for Pathfinder.