Disclaimer: As I write this I have fully read Lands of Creation and the guidebook to Meru, but have only skimmed through portions of Lords of Creation.

The guidebook to Meru is an excellent little book written as if were an actual artifact from Creation, the world of Exalted. It's written as a guide to newly exalted Solars explaining the capital city of Meru. It makes for a great introduction the First Age setting.

Lands of Creation is good for what it is, which is a look at selected bits and pieces of Creation in the First Age to allow you to get a feel for how the setting works. If you were hoping to get an idea of what your particular game's Second Age setting looked like in the First Age, you'll probably be a bit disappointed. A lot of the main areas are touched on, but only in broad strokes.

Lords of Creation is where most of the rules crunch is, and there's quite a bit to go through. I've only skimmed parts of it, but it looks interesting. One bit that we've imported into our Second Age game is the character creation rules for Allies and Mentors which are more detailed than those found in the core book. They require a little bit of tweaking for the Second Age, but have worked well so far.