Learning Photoshop mechanics and UX concepts isn't sufficient, IMO. One can be proficient in $DRAWING_TOOL and still unable to compose something that looks good. As much as I appreciate sites like Tutsplus, they teach mechanics but rarely answer the "why." Why do we add this 28% opaque drop shadow in step 37? Knowing why something looks good is essential to being able to create new works.
Two books I personally found very useful for "why," both discovered via HN:
"The Non-Designer's Design Book"
"Design for Hackers"
Two books I personally found very useful for "why," both discovered via HN: