What are some useful Pathfinder books for a GM?

[Gamerlord]

So I have had a decently long lull in my tabletop gaming hobby until recently, but now I have managed to assemble a group to play Pathfinder. I already own the Core Rulebook and Bestiary 1-2, and was looking to expand that but when I check out all the released books in the past four-ish years and it is rather overwhelming. Does anyone here have any recommendations? I'm the gamemaster, and all of my players are new to P&P gaming so I'm looking for something GM-focused.

[Skya]

for rules: advance players guide and advance race guide a full of nice options for characters.

If you want to play in the world of Golarion ( Pathfinder's official world), i recommand the Inner Sea World Guide. It's a campaign setting i really like and i highly recommand it.

[Squirrel_Dude]

NPC Codex, for when you don't have time to design your own characters.

[BWR]

It really depends on how much money you have to waste spend on the books.
If you need DM tips I'd go with the Game Mastery Guide. It's aimed at new DMs and most of the stuff will be known to experienced DMs and available in "help a DM" threads on various fora (stuff like world building, campaign building, adventure building, conflict resolution, purpose of rules, how to keep the game on track, how to use the rules, how to make good referee calls, etc.) but it is very concisely written and has a lot of useful but not critical stuff besides (NPC names, ship names, tavern names, plot hooks, and much more).
If you are interested only in mechanics I would suggest the Advanced Player's Guide first off, then Ultimate Combat, then Ultimate Magic. Advanced Race Guide is fun but not all that useful unless you intend to play races not found in the CRB. I'd pick up Ultimate Equipment before the ARG. The Advanced Class Guide is mostly a disappointment in my book, but that's because I dislike the concept hybrid classes (it can be done right but mostly it's just annoying). Lots of people like them however, but I wouldn't bother with if right off the bat if your players are newbies. Most of the mechanics of the game are easily available on the PRD so things like the Bestiaries and the NPC codex aren't nearly as necessary in dead tree format as the others.

[Miscast_Mage]

I've found the Ultimate Equipment guide quite nice. All the mundane and alchemical gear can really give a lot of flavour and variance to encounters, and make loot rewards more engaging to the players. Having a physical copy of the book is amazing to have to hand to new players so they can look at all the shiny toys they can play with! Having lots of pictures of the cool gear they can buy or find definitely has a wow factor on new players.

NPC Codex is useful, but you can definitely stick to using the SRD version of it and print out NPCs as you need them.

[atemu1234]

So I have had a decently long lull in my tabletop gaming hobby until recently, but now I have managed to assemble a group to play Pathfinder. I already own the Core Rulebook and Bestiary 1-2, and was looking to expand that but when I check out all the released books in the past four-ish years and it is rather overwhelming. Does anyone here have any recommendations? I'm the gamemaster, and all of my players are new to P&P gaming so I'm looking for something GM-focused.

The Advanced guides are always useful.

[Psyren]

Both Game Mastery Guide and Ultimate Campaign are chock-full of "GM tips." There's also a ton of great advice in the Pathfinder Society GM materials (http://paizo.com/pathfinderSociety/gmResources), specifically the "GM 101" and "GM 201" documents. Some of it is PFS-specific but the majority is usable anywhere, including things like problem players or improvising on the fly.

But all of that stuff is available free online and light on artwork. Personally I would save your gaming dollar for things you can browse through on the can for hooks and ideas - things like the Bestiaries, ISWG and NPC Codex.

Oh, and Inner Sea Gods - if you like Golarion, this thing is dripping with great fluff and mechanics.