If you want to master Photoshop, a complete course can be found in the books recommended below. Think of this as a multi-book master class.
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FOUNDATIONS
If you don’t already have a good, solid background in Photoshop start with Lightroom & Photoshop: The Essential Books before diving into the series of books listed below. If you are a true beginner, start here: The Best Digital Photography Books.
ADVANCED SKILLS AND TECHNIQUES
Your image processing begins with how you process your RAW files with Adobe Camera Raw (ACR) in Photoshop or Lightroom. Your best guides are by Jeff Schewe and the late Bruce Fraser. If you are using Photoshop CC or CS6 or using Lightroom 5 or 4, get The Digital Negative: Raw Image Processing in Lightroom, Camera Raw, and Photoshop by Jeff Schewe. If you are using an older version of Photoshop, get Real World Camera Raw with Adobe Photoshop CS5 by Fraser and Schewe (or the Photoshop CS4 version of the book or the Photoshop CS3 version of the book). Be sure to get the version of the book that matches your software version. You can find them all here. What you do with your RAW files determines how much or how little you can do with the file in Photoshop. You maximize your possibilities by making the right choices in ACR. Make the wrong choices, and you have limited what you can do later on.
After that, you can jump into color correction with the two books by Dan Margulis. I would suggest you read Professional Photoshop: The Classic Guide to Color Correction (5th Edition) first, followed by Photoshop LAB Color: The Canyon Conundrum and Other Adventures in the Most Powerful Colorspace.
Dive into digital retouching with the two books by Katrin Eismann. I would suggest you begin with Adobe Photoshop Restoration & Retouching (3rd Edition) before moving on to Photoshop Masking & Compositing.
It doesn’t matter whether you read Margulis or Eismann first, or if you read one book by Eismann and one by Margulis before moving on to the second book by either author, but I do recommend you start with a camera RAW book by Schewe/Fraser).
Reading these 5 advanced books, plus whichever basic-intermediate books you choose, will provide a master course in Photoshop. You will be able to do amazing things. Have fun!
If you want to go the extra mile (as if the above list of books isn’t enough, read Photoshop Channel Chops and Welcome to Oz.
Links
You can find all of these books in this section of my photography store which is powered by Amazon.com’s great prices, fast delivery, excellent service, and first class guarantee.
A more detailed description of the above books can be found in separate articles listed here: The “best of the best” photography gear, books, accessories, and online photo labs.