I believe that besides the Phobic coins hidden in each level, the only other is that after getting the Ninja power up in the credits level (Requires 100 Keys), and either completing or exiting the level, you'll retain the ninja power up on all the over worlds.
The book dark secrets 2 is not in stores yet. However, dark secrets 1 is.
2
Unearthing Ancient Secrets - 2009 Secrets of the Valley of the Kings 2-8 was released on: USA: January 2013
There are no secrets, execept getting new games.
The ISBN of Trading Secrets is 2-07-075231-3.
The ISBN of Age of Secrets is 0-921842-42-2.
Secrets of New York - 2005 Secrets of the Subway Above and Below 2-5 was released on: USA: 27 June 2006
Dead Men's Secrets - 2002 Secrets of Hitler's Wonder Weapons 2-3 was released on: USA: 19 September 2003
Secrets That Sell - 2007 2-10 was released on: USA: 2007
Secrets That Sell - 2007 2-11 was released on: USA: 2007
Secrets That Sell - 2007 2-12 was released on: USA: 2007
Secrets That Sell - 2007 2-13 was released on: USA: 2007