I haven't played 40k since 5th edition and I am looking to play my Necron army again and I am trying to figure out which of my books if any are still useful. It looks like none of my codexs are still useful for anything other than art (which makes since given I am 2 editions away from when I got them) but are the Apocalypse books still used? I don't see any Apocalypse books on the GAW website.
The most recent Apocalypse book came out in 2013, making it a 6th edition release. Since then, the 7th edition rulebook has already included many rules that were previously only mentioned in the Apoc book, such as Super Heavy Vehicles and Destroyers.
However, the Apocalypse book includes so many scenarios and Apocalypse-specific rules, formations and missions, the book is a must for you to run an Apoc game.
With that said, 40k: Apocalypse is still is in the GAW store.