I got some items for that I think, Rappan Athuk? and Barakus?
How much stuff is on it?
Could you give me a list of the different books (or at least a starting point). I think I found the ideas intriguing from what I read originally, but no idea how much more there is out there or where to start.
Thanks
FGG books are insanely large and detailed.
It is a rather kitchen-sink type campaign world and the world itself is less flavorful than the adventures. But it is designed to give lots of space for DMs to add their own content if they want, while still having so much material you really don't have to do much world building at all.
I would start with buying the PDF or physical copy of the Lost Lands Setting guide. See if you like the flavor. If you already use World Anvil, you could also try to subscript to the Lost Lands world in World Anvil. You can do that with a free World Anvil account but you do have to pay Frog God Games for the subscription. The cross linked articles, the interactive map, and search functions make it a great way to full up setting information in-game or when prepping games.
After that, there are a variety of massive regional and city setting guides and even their adventure books have a wealth of setting information for the various regions. Some of the more popular are:
Bards Gate - large, detailed book for the City of Bards Gate
Borderland Provinces - setting guide for a large European-inspired fantasy area on the Eastern cost of one of the two main continents.
The Grand Duchy of Reme - a plains people setting
Tehuatl (Inca/Mayan inspired setting)
The Northlands Saga (viking inspired setting)
Razor Coast (pirate themed, island hopping setting)
City of Brass - huge setting and sandbox adventure for the City of Brass (other plane - connected to plane of fire)
Cyclopean Depths (underdark setting)
and there are many many more.
You mentioned Rappan Athuk. I'm in the third year of running a Rappan Athuk campaign. This is just one mega dungeon located in one of the provinces from Boarderland Provinces. But the already massive book has many linkages so that players can go from Rappan Athuk to the Cyclopean Depths, or to the City of Brass, or to Bard's Gate, or other areas. For my Rappan Athuk game I have also regularly used the Bard's Gate and Borderland Provinces books for regional setting material. I've also referenced the Cyclopean Depths and City of Brass books although the party has not yet gone to either location.
There are some areas where the setting could collect information from all their various books, flesh out some more, and put them into one source book. In particular, information on the cosmology and religions. They've been talking about kickstarting a book on the Regligions and Gods of the Lost Lands for a while now, and I am eager for them to do this. I would rather have this at this time rather than the alchemy book and magic items book they've recently published.