The Adventurer's Guide marks a change in Paizo's policies, which now allow for Pathfinder campaign setting material to be included in Pathfinder RPG-line products that were previously setting-neutral, and for that material to also advance the setting's timeline. Among other conclusions, the results of this decision can include assuming that certain events from an adventure path take place with a specific canon result, or that characters did (or didn't) survive the adventure path.
As such, starting with the Adventurer's Guide these books are considered Tier 1 resources for setting canon, and because they are newer than the adventure paths that they reference, new or changed information about those adventure paths that appears in RPG-line books should reflect the current canon state of the setting on the wiki.
Pathfinder RPG-line books older than the Adventurer's Guide are not Tier 1 resources, and their contents do not necessarily reflect the canon setting. Creature statistics from Bestiary volumes remain the sole exception to this rule; bestiaries are still considered Tier 1 sources, but only for creature statistics.
Specific sources
What about the old Compleat Encounters? One of them is called The Vault of the Whispering Tyrant. Are they canon?
While some of these products refer to people, or places from the Pathfinder campaign setting, Paizo staff have stated here and here that they merely serve as inspiration for the current incarnation of Golarion. As such, these products shouldn't be used as source material for PathfinderWiki articles.
I heard that the information in Guardians of Dragonfall isn't valid any more... what's up with that?
It turns out that there was some miscommunication with the the author of that Pathfinder Module, which led to some material being included that didn't fit Paizo's vision of how dragons of Golarion should be portrayed. James Jacobs stated here and here that J2 shouldn't be taken as a source for information on Golarion.
Corrections and clarifications
Is everything said or written by a member of the Paizo staff about the campaign setting considered canon?
No. To quote James Jacobs about answers provided on the Paizo messageboards: "... keep in mind that answers provided via the boards here are not canon, since they're not printed and haven't gone through the same quality-control process that our printed words do."
PathfinderWiki acknowledges that venues like the Paizo messageboards provide fans with useful places to request corrections and clarifications about published canon works, but the wiki intentionally designates that only "verifiable statements from Paizo editorial staff that correct or clarify specific canon setting details" are canon, and that those details do not override any other published canon works.
Is a retcon a correction or clarification? Does a retcon create a canon conflict?
PathfinderWiki handles deliberate, intentional retroactive changes to canon continuity, also referred to retcons, differently from official statements correcting content errors or clarifying unintentional canon conflicts. Retcons are not part of the tiered canon source system because handling them often requires more subjective consideration than a tiered system with prescriptive outcomes can provide. They also benefit from more guidance on communicating those changes than resolving a canon conflict, and might require handling them uniquely and with consensus in order for the wiki to be made and remain internally consistent as a result of the change.
