If you want to master Photoshop, a complete course can be found in the books in the posts below.
Begin with one or two of the basic to intermediate books in the next post. After that, jump into the list of advanced books in the posts below.
ADVANCED SKILLS AND TECHNIQUES
Your image processing begins with how you process your RAW files with Adobe Camera Raw (ACR). One of your best guides is Bruce Fraser and co-author Jeff Schewe. What you do with your RAW files determines how much or how little you can do with the file later on in Photoshop. You maximize your possibilities later on 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 your start with Fraser and Schewe.
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!