I've found this good website to found an reference ebook on
Photograpy.

http://dl.mazudi.com

Here are some free photography related ebook I found when search on
the site:

A Short Course In Digital Photography
A Short Course in Digital Photography (pdf)
Advanced Photography (pdf)
Basic Flash Photography (pdf)
Beginners Guide to B+W Photography (pdf)
Close-Up Photography With 35mm Cameras
Course Technology - Mastering Digital Photography, 2nd Edition, 2005
Digital Outback Fine Art Photography Handbook (DSLR Guide)
Digital Photography - 99 Easy Tips To Make You Look Like A Pro!
Digital Photography - Acquisition And Processing Techniques By Ron
Reznick
Digital Photography - The Digital Darkroom (pdf)
Digital Photography - Tips & Tricks
Digital Photography Bible
Digital Photography Just the Steps for Dummies
Digital Photography Tips
Digital Photography Workflow Handbook (pdf)
Digital Sports Photography
Exploring Digital Photography (pdf)

And more...