Hello!
I am slowly expanding my collection of Pathfinder books, but I'm not sure what to prioritize/what is worth getting.
I currently have Core, APG, UM, UC, and the ACG. I am planning on getting Bestiary I.
I am a player and now a GM, so I'm interested in almost everything, and my group has a policy of only using material that we own physically.
I have considered getting the GM's guide, but I'm not sure if I really need it. It's on the PRD, and a friend owns it if I really need physical access.
Which books should I get next, sticking to Pathfinder products?
I'm mostly debating Ultimate Equipment, Ultimate Campaign, Advanced Race Guide, and tentatively the Inner Sea World Guide and the GM's guide (though I'm not sure I really want to invest money in it).
What is your advice? Which should I prioritize? Why?
Ultimate equipment and the inner sea world guide are both fantastic. The others are good, but those two are amazing, in my opinion.
Once your have the Beastiary, you will have all of the "most core" books. Anything after that will be a matter of where your group's interests lay. If your game is set in Golarion, then pick up the Inner Sea Guide. This campaign world doesn't have the depth (yet) of other more established settings like Forgotten Realms, Ebberon, and Grethawk, but Golarion is pretty diverse with lots of flavor. Just about anything can find a home there.
I just started playing Pathfinder a few months ago and my shelf is already covered with books. If you're mostly just participating as a player, I'd recommend getting the Ultimate Campaign or Inner Sea World Guide next. Ultimate Campaign has a great section on creating your character's background and what they do during their downtime. The Inner Sea World Guide is the best all-in-one sourcebook for the world of Golarion. I love reading through sections and figuring out where my character came from and what their goals should be.
Also I can't recommend enough the various Players Companions that appeal to you. They offer an equal amount of fluff and game crunch to add to your character and they always serve as great inspiration when thinking about my character. I love the Pathfinder Society Primer, Undead Slayer's Handbook, Dungeoneer's Handbook, DragonSlayer Handbook, Demon Hunters Handbook and so on and so forth.
Conversely, if you're going to focus on being a GM - you STILL need the Inner Sea World Guide so you can fill in the world your characters are playing in and bring it to life for them. AND you need as many of the Bestiaries as you can afford. You can never have enough monsters! Once you get the Paizo bestiaries, go for the Tome of Horrors Complete, Tome of Horrors 4, Dreamscarred Press' Psionic Bestiary, and Fat Goblins' Mindblast! Classic Monsters Augmented.