My guess is that the Adventurer's Handbook will be pure splat; and that those hoping for a splat-free edition will be sorely wounded. In general, I hope the edition sees some splat, some of it tied to specific settings and their adventure paths, some of it of a more general nature. I actually think some modest degree of splat will be good for the game and good for the market. I'm not going to buy toys, lunchboxes, or even the MMO that crushes WoW beneath it's heel. I won't buy minis (I already own and play a couple of versions of D&D that don't play well without them; and 5e has promised to be my TotM edition), cards (collectible or otherwise), maps, or tiles. I will however buy campaign settings, adventures, and splat. All of that said, I really do hope there is less splat than we saw for previous editions, just not a complete absence of it.
Now, as for the other book-- is it Princes of Darkness or Princes of the Apocalypse. My guess is that-- whichever it is-- it ties to the story arc that will follow ToD. If Darkness, I bet we are getting some Raven loft action. If Apocalypse, it could point toward Dark Sun.