Does the Terrinoth book have much fluff and detail on the setting?

what are some good resources that provide that detail?

I feel like the majority of the book is setting material, and that it does a good job of painting the broad strokes of the setting.

Yes, as Cantriped states it has quite a lot of setting material. I didn't really know a lot of the setting before I bought the book but actually found myself really drawn in by the lore and the possibilities.

From what I can gather the RoT book is the only compendium of the world of Mernara. No other product in the past has laid out the world so clearly and this is likely the document that has become the definitive Canon for FFG in this world. Basically this book says everything there is to say, excluding only the specifics of stories from the Descent Adventure paths.