Blend Modes
https://helpx.adobe.com/photoshop/using/blending-modes.html

Composite Examples: a round up
http://www.flickr.com/groups/photocomposites/

http://petapixel.com/2012/06/19/time-lapse-of-a-plane-crash-composite-photo-being-created-in-photoshop/

Joel Grimes: http://youtu.be/1c0_pLKtG5Y

http://thephotobrigade.com/2013/02/composite-photography-by-tom-di-maggio/

http://digital-photography-school.com/a-guide-to-create-eye-catching-composite-images

http://www.mediabistro.com/10000words/composite-photography-picture-is-worth_b322

http://maggietaylor.com/

http://www.uelsmann.net/

http://www.dominicrouse.com/

Books to explore:

Photoshop Compositing Secrets: Unlocking the Key to Perfect Selections and Amazing Photoshop Effects for Totally Realistic Composites [Paperback] Matt Kloskowski (Author)

http://www.amazon.com/Photoshop-Compositing-Secrets-Selections-Composites/dp/0321808231

Layers: The Complete Guide to Photoshop’s Most Powerful Feature (2nd Edition) [Paperback] Matt Kloskowski (Author)

http://www.amazon.com/Layers-Complete-Photoshops-Powerful-Feature/dp/0321749588/ref=sr_1_1?s=books&ie=UTF8&qid=1378229456&sr=1-1&keywords=layers

Adobe Master Class: Photoshop Compositing with John Lund Paperback, by John Lund

http://www.amazon.com/Adobe-Master-Class-Photoshop-Compositing/dp/0321205456/ref=sr_1_5?s=books&ie=UTF8&qid=1378229558&sr=1-5&keywords=photoshop+compositing

The Hidden Power of Blend Modes in Adobe Photoshop by Scott Valentine
http://www.amazon.com/Hidden-Power-Blend-Modes-Photoshop/dp/0321823761/ref=sr_1_6?s=books&ie=UTF8&qid=1378229846&sr=1-6&keywords=photoshop+compositing