Steam lists two versions of this game: Cities: Skylines and Cities: Skylines Deluxe Edition (and also the standard to Deluxe upgrade pack). Unfortunately, it doesn't actually say what the difference is between them. What is the difference?
I found a listing for it on Green Man Gaming that lists the Deluxe assets as follows:
Included in the Deluxe Edition are 5 In-game historical monuments from around the world, the games original soundtrack as well as a digital art book.
Five in-game items include:
Statue of Liberty
Arc de triumph
Grand central terminal
Original Soundtrack: This Original Soundtrack includes 14 unique tracks mixed from the ambient music of the game, allowing you to enjoy the wonderful music whenever you want.
Digital Art book: See the concepts behind the buildings! The book features almost a 100 hand drawn concepts of the game various buildings and the story behind each.