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Thanks for your question. You'll find the Missing CD page for OS X Yosemite: The Missing Manual here:
Here's the direct link to the Photos PDF appendix (listed on the Missing CD page under the Chapter 11 heading): http://bit.ly/1dHD6IR
To answer your other implied question, this appendix is limited in its scope to an introduction to the program. If you're looking for something more comprehensive, you might also be interested to know that we're currently working on a full Missing Manual about the Photos app for Mac and iOS: http://shop.oreilly.com/product/06369...
Senior Editor, O'Reilly Media