So I'm thinking of starting a Force and Destiny campaign. I already have the core rulebook and GM kit.

Which of the sourcebooks would you recommend I get first?

It mostly depends on what kind of group you are running for and what kind of campaign it will be, so knowing that would help answer the question. The sentinel book is useful for urban, high tech campaigns, while the seeker book helps with more primitive, wilderness themed campaigns. The consular sourcebook is very focused on the force itself, and provides information on various force traditions. The guardian book is very useful for combat focused campaigns. Guardian and sentinel also provide rules for crafting armor and lightsabers, respectively, if any of your players are based around crafting (special modifications in EotE helps too), the seeker book is great for players who want to control beasts or have a pet, and consular helps players who focus primarily on the force itself. Hope this helps.

Everything ?

More seriously, Nexus of Power or 1 of the pre-written adventures, Chronicle of the Gatekeeper or Ghost of Dathomir. Eventually the soucebook of each of your players careers would be a great idea.

Get what is relevant to your group make up. Fnd is very flexible as a narrative game. If a rule does not fit your needs.... Make one up.

Whatnkind of F&D game donyoh want to run. For example a game with characters investigating and rigjti g wrongs would benefit from Endless Vigilance. A story out in the wilderness would benefit from Savage Spirits.

Yes as everyone has stated; the books all depend on the campaign style. With you stating that you're starting with Force and Destiny it would seem likely that your party is interested in Light Sabers, The Force and either being Jedi or Sith.

My favorite Force and Destiny books, in no particular order would be.. Nexus of Power, Endless Vigil, and Keeping the Peace. These books give a great mix between information on locales and great crafting rules.

Endless Vigil is the Sentinel source book. Bringing great options for crafting light sabers and even Pod Racing! The player options within in are great too.

Nexus of Power gives a bunch of information on planets strong in The Force. It also gives you rules in information on creating your own Vergences.. Think Empire Strikes back when Luke gors into the cave on Dagobah and sees Darth Vader.

Lastly with Keeping the Peace, we get new options for armor crafting, great player options, and some fun gear to throw into the mix.

Hope this helps!

Nexus of Power is like a chest full of toys to play with. I've spent nearly 17 months living in Chronicle of the Gatekeeper

Yeah, everything. But if I were you I'd actually start with the other two core books. It's a big game, and everything is compatible.