[2019] DIFFERENCE BETWEEN STANDARD & DELUXE ?

You also get the soundtrack and the art book. Nothing secret in-game. At least, nothing that I know of. But it wouldn't be a good secret if I did.

From the Store Page: The Deluxe Edition includes the base game, official soundtrack, digital art book, two masks for PAYDAY 2 and three keys for the upcoming closed beta. If you buy the 5-pack, you get five copies for the price of four - perfect if you are a group of friends who want to play together and want the complete experience.