If you're looking for guidance specifically for Photo Manipulation (as opposed to photo editing/retouching etc.) I would recommend getting one (or both) of these books:
Amazon.com: How to Cheat in Photoshop : The art of creating photorealistic montages - updated for CS2: Books: Steve Caplin
Amazon.com: Surreal Digital Photography (One Off): Books: Barry Huggins,Ian Probert
I have both these books and they teach a massive amount about manipulation techniques. I know Photoshop books may seem expensive, but they represent a good investment.
If books aren't to your liking, the best place to look would be the big tutorial sites: most have a section on Photo Manipulation tutorials.
Photoshop Tutorials / Good-Tutorials
Photoshop Special Effects Tutorials - Search for Photoshop Tutorials on Pixel2Life