Sunward and Rimward both cover locations and politics in areas of the solar system. Sunward (sun to asteroid belt) covers a lot of the inner system. Rimward (asteroid belt and beyond) covers a lot of outer system stuff. Gatecrashing. Covers locations and politics beyond the Pandora gates. Also how to do gatecrashing professionally. Panopticon. Covers a lot on how societies works in AF 10. Also a bit on uplifts. Transhuman. A good book on pushing the limits on transhumanism. Has rules for using exotic morphs like flexbots and swarmanoids. Introduced Eidolons, morphs for informorphs. Experimental psychosurgery and merging forks. Revising some other rules. Etc. Firewall. It covers a lot about Firewall. I don't remember much about it at the moment. X-Risks. A book about a lot of dangers Firewall has already faced. And some they have yet to see. You'll might think everything is dangerous after reading this book. ---- I would highly recommend Transhuman. There were a lot of ideas introduced in the core rulebook that weren't very well explained. The core rulebook might say that flexbots are shape changing combining robots, but no hard rules were provided until this book. I think this is my favorite book for Eclipse Phase.

…Followed by Panopticon. A lot of the stuff in Rimward, Sunward, and Gatecrashing is *really good* but Panopticon covers some of the stuff that's easiest for players to miss, or have difficulty wrapping their heads around. How a society with ubiquitous surveillance feels and works, for example, has fewer touchpoints in pop culture and scifi than many of the other concepts.

Here's a link where you can download the PDFs for free. https://robboyle.wordpress.com/eclipse-phase-pdfs/ If you like them, I suggest you buy a few.

Transhuman and X-Risks are really the absolute best and the absolute musts, to my mind; Transhuman because it provides the most tools to players and gives you much more to play with, X-Risks because it gives you way more to work with in terms of adversaries. All the books have their use, though. The system survey books (Sunward, Rimward) are both useful as ground laying, and Gatecrashing is great if you want a more adventure-oriented game or a really exotic locale that's more sci-fantasy than sci-fi.

I agree with the idea that you should look at the pdfs first, they're free which is a blessing for students like me (the fact I could legally download a free pdf convinced me to spend money on the core rulebook before reading it all, despite having a limited budget). Then my suggestion is to buy the book you think you might use first. I'm going to agree that Transhuman is a great choice, because of it's clarifications and alternative character creation systems (I love the default system but not everybody wants to spend 1000 points to create their character). Otherwise it depends on what you want, each has a separate function and you'll likely want different books depending on if your campaign is focused on Firewall, Gatecrashing, Hypercorp troubleshooters, anarchists, scum, or whatever your idea is.