The world book is actually set before the series begins, and describes the world at that point and its history leading up to that. The conceit is that it's an actual in-Westeros book given to Robert Baratheon some time after he became king but (obviously) before he died and AGoT takes place. The new information will be expanding rather massively on historical events, including the Dance of Dragons, War of the Ninepenny Kings, Blackfyre Rebellions and Aegon's invasion.
I suspect the 'GRRMarillion' will delve much more into what would now be considered spoiler material, particularly about the Children of the Forest, Others and ancient backstory (which is what The Silmarillion was to LotR, after all). However, that's also (I imagine) why it's coming out after the series is done, because then GRRM doesn't need to hold back on anything.
This is pretty standard: the Wheel of Time companion book described the situation just before The Eye of the World began. The Shannara book I think was set in a gap between two of the later series, but then Shannara isn't one massive story. Riftwar's companion book came out after the series was done, but was also crap.
Edited by Werthead, 27 November 2013 - 11:42 AM.