Most books on PS are full of great tips and if only just starting out then any of the Photoshop for Dummies or Idiot's Guide are great.
They give step by step instructions which is a real help because PS is very intimidating at first.
These basic principals can be carried over to astro image processing.
Once the basics are learned then more advanced techniques can be used for astro imaging.
One site that is often quoted is Jerry Lodriguss'
"Catching the Light".