Image courtesy of Paizo
The Pact Worlds resource for Starfinder is 215 pages of more everything. Looking for a few more races or themes for your characters? It has that. Looking for new items and star ships? It has those too.
The bulk of this book though is campaign setting, that is of Starfinder’s own solar system, the Pact Worlds. Covering the fourteen locations found on the inside of the cover of the Core Rulebook, Alien Archive, and now the third resource for Starfinder; Pact Worlds goes into a fair amount of detail of each location, covering climate, populations, politics, history, notable locations and even a few ideas for new themes for characters that would make sense coming from these places. As we have come to expect with this system, Paizo has made sure to include beautiful artwork and maps as well as consistent organization and aesthetic, making this resource feel just like an expansion of the other books released so far.
I have found the little bit of history of persons of interest, such as Ceris Hightower, the longest serving member of the Pirate council of the Diaspora, to be well developed, making me want to use them for interactions with my players. She is but one example of so much that Pact Worlds has to offer if you are looking to expand your library. The book delves into the solar religions of the Sun and the various structures and AI rulers of Aballon, the planet of robots. How and why of the great immortal Liches of Eox, the drow of Apostae, the great corporate Dragons of Triaxus and their trade networks, and so much more.
If you are looking for more of Starfinder and haven’t already picked it up, give it a look. At the least it may give you ideas for your games, or new races if you like to have options.