I don't think this is a study of the tools (Photoshop, et. al) it's more a study of light, perspective, and traditional artistic techniques. Regardless of what you use to create an image, you need to understand how light works, how texture works, how the perception of depth is created, etc.
If you know the basics of Photoshop, it's merely a matter of practice to apply techniques in a manner which more closely mimics reality. Even though there may be 10 different methods for creating a particular aspect of an image. If you know how to create a shadow, then that's all there is not know about the tool. Beyond that it's a matter of creating a shadow at the right angle, the right depth, the right darkness, etc. and do all that consistently across the entire image, you're on your way. All of this is not really related to any particular tool. A great many users simply go too far with effects or painting and destroy any fabricated reality.
I've always found books by Burne Hogarth to be very helpful in learning tricks for realism from a fabricated standpoint. Even though many of the books focus on figure drawing, the ideas contained in the books translate very well to any shape, not just human figures. I'd especially look at the books relating to light and shade.
I'd also suggest practicing with a pencil and paper which more easily allows immediate exploration.