From the comics suggested from the past few pages Trinity, Green Lantern, Flash and Justice League all sound good.
Green Lantern Rebirth TPB: it seems Amazon only have the hardcover for this. Also, is it just a single volume?
Green Lantern (ongoing): should I just go for the more recent stuff?
Johns run on Flash: by Johns you mean Geoff Johns? There seem to be a few so which one is the starting point?
Giffen/De Matteis on Justice League: I've found more recent Justice League International stuff by them, in hardcover also.
Thanks for the suggestions so far!