I answered only FR, because it's got so much material available from older editions, including free articles online, that it's my idea of a DM's paradise.
More important than any of the others is information on building your own setting.
I could also see a setting conversion book that would help rewrite the mechanical parts of any previous editions' campaign settings. This would include pantheons and cosmologies, to the extent that they affect playing clerics or casting some spells. Also, refluffing and tweaking races and classes to fit those settings.
More important than any of the others is information on building your own setting.
I could also see a setting conversion book that would help rewrite the mechanical parts of any previous editions' campaign settings. This would include pantheons and cosmologies, to the extent that they affect playing clerics or casting some spells. Also, refluffing and tweaking races and classes to fit those settings.