Secret Empire 0 pulls together threads from several different books which constitute the prologue. They are conveniently laid out here:
I only read the Steve Rogers issue and didn’t really feel like I needed to read the rest to follow what was going on in Secret Empire 0. In Steve Rogers 16 we see Cap’s secret plan start to coalesce as Hydra seeks to reassemble the Cosmic Cube without reawakening Kobik and Zemo appears to kill Bucky.
This brings us to Secret Empire 0 wherein it is outlined that the Cosmic Cube was apparently created by American scientists during the World War II era and used to create the reality that we know as the original Marvel Universe. Hyrdra’s plan is to use Cap to “undo” that “fake” reality and restore the “true” reality in which Hydra conquers the world. So they appear to be telling us that the entire Marvel Universe is all just one big EYKIW.
On the other hand, we are still getting the intro text page telling us that Kobik has rewritten reality. In which case this whole Captain Hydra saga is just one big bad dream that can be Bobby Ewinged out of existence at any time.
In the meantime Nick Spencer has started knocking down the dominos that he has nicely put into place over the last year or so. This isn’t just a bunch of random stuff thrown together to sell a summer crossover. There appears to have actually been a lot of planning that went into this.
Here’s the way it will unfold going forward:
It doesn't appear that there are too many tie-ins or spin-offs, although I suppose that could change but so far I'm on board for the main series and probably Uprising as well.