He's currently writing it and hasn't released it yet, however you can check out this thread: Ask a Derpling anything.
Terraria Community Forums
The Storm
Dapling
I’ll invite you to the Derplore conversation later, all of the big important stuff is on the very first page.
Enchanted Sword Ω
can you invite me?
Dapling
I guess.
Ivyss
I'm just gonna ask that you give credit to anyone that's contributed, be it through through ideas or otherwise.
Dapling
Ok. That's really only @Sir. Fluff though. I actually came up with most of it due to my sheer insanity. 
Ivyss
My main reason for not giving you much help is just that I'm making my own world, which I have no direct base for like you do.
Dapling
The Official Lore is actually like half as fleshed out as the Derplore, and a lot of it was basically made up, so Terraria is less a base and more an "end result."