If someone were interested in Starfinder, the first three books I'd recommend they buy would be the Core Rulebook, the Alien Archive, and Pact Worlds. The first one gives the rules, the second one gives the adversaries and allies, and the third one gives the setting. Pact Worlds is a 216-page hardcover book primarily intended to provide a detailed overview of each of the planets in the setting's main star system. In addition, the book introduces several new starships, NPC stat blocks, and some player options like new archetypes, playable races, and weapons. The artwork and layout are really pretty, though the book is a bit pricey given its length. Still, I consider it a nearly essential buy for GMs looking to add depth and richness to their Starfinder games.

The book starts with a four-page introduction, "Welcome to the Pact Worlds", that provides a nice, readable summary of the general government, economy, and culture of the system. Some key historical events are put into perspective, which is something I appreciated. For example, the alliance with the Vesk is a relatively recent phenomenon after decades of war, and there are still some tensions there. The description of the Stewards was interesting, and one gets the sense that they're essentially Jedi Knights, roaming the galaxy and settling local disputes. Economy-wise, it seems capitalism runs unbridled, which probably makes for a good setting for adventurers even if it's a bit depressing to imagine.

Chapter 1, "The Worlds", weighs in at 140 pages and is by far the longest chapter. Each of the planets in the Pact Worlds receives ten pages of coverage broken down into the subheadings of Geography, Residents, Society, Conflicts and Threats, and Notable Locations. A one-page map of each planet is provided, but there's something about them that just doesn't look right to me. Each world-entry then concludes with a new character theme suitable for PCs from that planet. Here are the worlds covered:

* The Sun: Far from simply a source of light, heat, and gravity, the Pact World's sun is the home to the "Burning Archipelago", a collection of "force bubble" cities and tunnels of mysterious origin. The various cities are controlled by different groups, and there are several mysteries (such as why the lashunta feel psychic emanations of paranoia, or whether institutions like the "Sun Atlas" or "Deep Cultures Institute" are correct that there may be civilisations within the sun itself). Overall, it's a clever way to make the sun itself an interesting place to have adventures. Now that I'm preparing the Dawn of Flame adventure path, I appreciate it even more. As for the theme: the Solar Disciple has benefits so minor as to be completely forgotten by most players.

* Aballon: A world of megacities inhabited primarily by anacites--sentient robots. Aballon is a major manufacturing center in the Pact Worlds. Although I still don't have a very good picture of what individual anacites are like when encountered as NPCs, there are some surprisingly interesting plot hooks written into this section: the mysteries of the First Ones, the political ramifications of the Machine Court, or the riddle of PreceptumXIII, a megacity constantly built and destroyed. I could actually see a campaign here. The new theme, Roboticist, is one I've actually used for a character and it's reasonably good as far as themes go.

* Castrovel: A planet with an interesting mix of continents occupied by lashunta, formians, and elves, as well as an array of wild, dinosaur-sized beasts. There are some potential political/intrigue plot threads a GM could work with, given that a war between some of the species just ended. Stories involving any sort of wilderness theme could easily be set here, and setting elements like the Green Faith, the Xenowardens, and more are incorporated. I have some fond memories of adventuring on Castrovel with my barathu envoy, and I wouldn't mind heading back. The new theme, Wild Warden, has very useful and fitting abilities. It's a good model for what themes can offer when done well.

* Absalom Station: This is the most crucial location for most GMs and players, as it's a sort of default setting/home base for a lot of adventures (both adventure paths & Starfinder Society games). Unlike some of the other maps, I do like the top-down view we get of the station here. Absalom Station has some important links to setting lore (the Gap, the Starstone, the Drift), but there's also some good discussion here of local politics and problems: everything from income inequality to gerrymandering to a "Strong Absalom" movement with unfortunate real-world analogues. I particularly liked the Eyeswide Agency (providing psychic investigative services) and the Simar Communion (identical clones who work as spies and assassins!). The new theme is Corporate Agent, which is a good concept and executed in an okay way.

* Akiton: This is the Mars or Tattooine analogue, a difficult rocky desert wasteland in decline. It's perfect for some Firefly-style space cowboy adventuring. An interesting mix of 5 species (hylki, ysoki, contemplatives, ikeshti, and shobhads) inhabit the planet. There's a ton of great little adventure hooks littered throughout the section, such as extensive wrecks left sometime during the Gap, a flying tourist attraction called Five Tines Fortress, and (in a shout-out to an early SFS scenario), old thasteron mines. The new theme is Gladiator, which is okay.

* Verces: The most technologically advanced of the Pact Worlds, Verces is a place where human augmentation is extensive. It's also the politically most advanced world, and served as the model for both the Stewards and the Pact Worlds system of government. I didn't quite get a picture on what adventuring on the planet would be like, however. The new theme, Cyberborn, is rarely going to be meaningful in practice.

* Idari: This world-ship was the means by which kasatha came to the Pact Worlds. There's a surprising number of lakes and variety of ecosystems within the vessel. A number of factions are in play in the Idari, which raises several natural story possibilities. I like the feel of the place and there's a lot of material to work with, though there are few obvious adventure hooks. The new theme is Tempered Pilgrim, which is what kasatha are called when they go on a yearlong walkabout to learn about new cultures. It has pretty good features.

* The Diaspora: A huge, lawless asteroid belt full of tons of great adventure hooks. My favourites are the Abattoir and the Hum, both classic science-fiction/horror set ups. If you're interested in pirates, dwarves, or sarcesians, the Diaspora is the place to be. I liked the write-up for Songbird Station and the shout-out to Strawberry Machine Cake. It's nice to see crossover with SFS scenarios. The new Space Pirate theme has some pretty useful abilities.

* Eox: This is a cool concept: a planet ruled entirely by intelligent undead after an apocalypse killed most of the living. There's good places for exploring (like the Lacustria Sea), plenty of opportunity for random encounters (from feral undead), room for political machinations (with the Bone Sages), and the classic reality-t.v. horror of the Halls of the Living (plus Zo!). I've used the Death-Touched theme in a game, and it was fine.

* Triaxus: Triaxus is one of those concepts that sounds interesting at first: a planet that has an erratic orbit, meaning winters last centuries and then summers last centuries). But in practice, PCs are only ever going to encounter the long winter season unless they travel through time or something. The natives are ryphorians, and dragons are a common feature (with dragon-kin sort of in-between). There's frankly not much here that I found exciting or that stuck out at me. The Dragonblood theme looks solid, however.

* Liavara: Formally a protectorate (not an independent member of the Pact Worlds), this gas giant proved more interesting than I expected. Some of its settled moons like Arkanen and Nchak have potential, and the adventure hooks here are solid. The Dream Prophet theme isn't bad, especially the 12th level ability.

* Bretheda: Another gas giant, but this one is home to my favourite floating jellyfish aliens, the barathu. Its moons hold kalo, haan, and some other important species. The "kalo fashionista" pic on page 123 cracks me up. I like the story seed with a creeping, fatal fungal infestation among barathu called the Laoe Araae that threatens to spread to other species. I could see an adventure involving finding a cure for it. I also like the moon Yashu-Indiri, in which monks have built shrines to abandoned, dead, or otherwise forgotten deities--including those that received widespread worship prior to the Gap, like Torag and Cayden Cailean. The new Biotechnician theme is solid.

* Apostae: Drow! This artificial planetoid has a (mostly) inaccessible interior, and there's plenty of story to be had in PCs trying to figure out how to open one of the doors (and then exploring whatever fantastical things it leads to). Special shout-out to Nightarch, site of an epic SFS scenario I ran that lasted until 3 a.m. and led to an 80% TPK. Good times! The new Xenoarchaeologist theme has probably the best special ability of any theme I've seen: Trap Spotter (just like in Pathfinder, a free chance to spot traps just by walking near them).

* Aucturn: This planet is not for tourists. It's a mutating, pulsing, toxic planet that can change to fill whatever needs the GM has for it. The site of battles between the Dominion of the Black and cultists of the Great Old Ones, this is a planet with a clear Lovecraftian theme. Mad recluses, warlords, monstrous things, and more are here. The image of Carsai the King is very cool. I'm not a fan of the new theme, Cultist, as it comes across as very generic.

Chapter 2 is "Starships" and comes in at 16 pages. We get a better explanation of the Drift, though I still find it's a feature under-utilized in the system (and I still wonder whether communication in or out of the Drift is possible). Some new starship options are introduced, such as biomechanical ships (simple but cool), drift shadow projectors (akin to interdictor ships in Star Wars), and more. Overall, I like the new options--some are necessary "bread and butter" ones left out of the Core Rulebook, while others show some good creativity. Stats (and good artwork) are given for new ships for Aballonians, Hellknights, Iomedaeans, Vercites, and Xenowardens.

Chapter 3 is "Supporting Cast" and also weighs in at 16 pages. This is effectively like a mini-NPC Codex. It contains stat blocks (and alteration suggestions) for a variety of NPCs that a GM might need at a moment's notice: Cultists, Free Captains, Hellknights, Mercenaries, Security Forces, and Street Gangs. I find material like this incredibly useful, and I like how it also saves space for writers of SFS scenarios and APs.

Chapter 4 is "Player Options" (35 pages). There's a lot here: new archetypes, feats, weapons, armor, items, spells, and races.

Six new archetypes are introduced here, though (unfortunately, to my mind) no actual affiliation with the related organisation is necessary in order to take them. The six are Arcanimirium Sage (really good, and perhaps most interesting for non-spellcasters), Divine Champion (kind of bland, though Divine Judgement is very useful), Skyfire Centurion (some good abilities, though it seems to demean the bonding process), Star Knight (interesting and I like the variations), Starfinder Data Jockey (bland and makes it easy to use an already overpowered skill, Computers, in place of other skills), and Steward Officer (Demand Surrender is too late and too weak).

Eight new feats are introduced. Some are fairly underwhelming (like the stage magic line), but some of the Divine Blessings (which vary depending on which deity worshipped) are pretty awesome (one lets you overcome immunity to a particular type of energy!).

Regarding the new equipment, there were a few things that stood out to me. Some of them have become almost commonplace in games, like magboots and library chips (way overpowered for their trivial cost). Others just look fun, like magical energy drinks and motospheres. This is the section that allowed my descent thruster-equipped character to *very slowly* fall into a pool of acid and die horribly.

As a preface to the new spells, a little blurb explains who and how they were created (in-game). I loved this, and wish we got more. It really adds to the flavour. Anyway, this section introduces the "junk" line of spells and a few others. One of them, control atmosphere, would definitely have its uses.

Finally, six new playable races are introduced: astrazoans, bantrids (cool origin and flavour), borais, khizars (kind of bland), SROs, and strix. I've actually played two of these.

And that's the book! It's not perfect, but on the whole I think it's an excellent overview of the setting and provides a lot for both GMs and players to take advantage of. I'd definitely recommend it.