To get a good feel for the world, after reading the Core book, what are some other rulebooks that tend to give you the most flavor?
Something in the ink from the old 3.5 books makes them taste the best. But the Pathfinder Chronicles: Gazetteer with its faint smokey flavor and hint of sourness is a real blast of opulent flavor for your tongue! As Caedwyr said above, I would have to agree The Great Beyond and Gods and Magic are tasty as well...
For me it's
The Great Beyond, Book of the Damned vol. 2, Guide to Absalom, PF 36, City of Strangers, Heart of the Jungle, Into the Darklands
I don't have a vast collection, but I'm a sucker for city guides. My campaign is very Varisia-focused, so I've mostly bought Campaign Guides and Players Companions that are focused on that area.
I absolutely love City of Strangers and Magnimar: City of Monuments. I like them so much that I'm running a bunch of sidequests in both cities in my "Rise of the Runelords" campaign, just so that I can keep more action in those places.
I picked up Lost Cities of Golarion the other day, thinking that I could use the Xin-Shalast info in my Runelords game. It turned out not to be so useful for the AP (it details the ancient city assuming that Runelords was completed successfully), but I think the book was worth it just for the city of Ilvarandin! That place is amazing... it's Invasion of the Body Snatchers crossed with The King In Yellow!
I also managed to find a copy of Distant Worlds at a used bookstore, and am really digging the ideas of sci-fi tropes in a fantasy context.
As for players guides, Cheliax: Empire of Devils is an interesting read, although as a Players Companion doesn't go into as much detail as I would have liked. But the one I really like is the recent Varisia: Birthplace of Legends. Especially the amazing two-page players map. That map alone is worth the cover price!
I don't have a vast collection, but I'm a sucker for city guides. My campaign is very Varisia-focused, so I've mostly bought Campaign Guides and Players Companions that are focused on that area.
I absolutely love City of Strangers and Magnimar: City of Monuments. I like them so much that I'm running a bunch of sidequests in both cities in my "Rise of the Runelords" campaign, just so that I can keep more action in those places.
I picked up Lost Cities of Golarion the other day, thinking that I could use the Xin-Shalast info in my Runelords game. It turned out not to be so useful for the AP (it details the ancient city assuming that Runelords was completed successfully), but I think the book was worth it just for the city of Ilvarandin! That place is amazing... it's Invasion of the Body Snatchers crossed with The King In Yellow!
Do you have the guide to korvosa? It's one of my favourites (and it got a pretty high number of five star reviews).
I found Land of the Linnorm Kings to be one of the best I've read in years. Mythic and awesome.
Faiths of Purity/Balance/Corruption: This really helps you get a handle on what worshiping a deity means from a character perspective.
After the Faiths, whatever book covers the area you intend to play in. Linnorn Kingdoms is indeed well done. River Kingdoms is my favorite though. Outsea is the greatest!
Distant Worlds: It gives you an idea of how Golarion fits with everything else.
Books of the Damned: Explains evil very well, and gives some nice background tidbits on why Sarenrae and Asmodeus can cooperate.