Astulae is a very different sort of sourcebook, dealing with a very different sort of setting. Worlds crashing into each other, blending and twisting, with a very simplified game engine to tie it all together. Honestly, it's weird, and I love it.
If you're running games using the ADRPG system (Amber, Lords of Gossamer and Shadow), this book is a must. It provides a plethora of worlds, and a puzzle for the characters to try to solve. Is this a natural phenomenon? Is this a problem that needs to be fixed? What caused it, what does it mean for the countless worlds untouched by it - when it is only a matter of time before it swallows everything? You can build multiple campaigns on this book!
As a stand-alone? I think it's solid. It provides the means for making your own worlds, it provides means for hooking them all together, and it gives you a straightforward system for making characters which you can use if you don't have another game engine you're already using. It can stand on its own strengths, or be used to enhance any other game.