Is Apocalypse still a rule set in 7th edition and are the books still useful
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.