Art, at the core level, can't be taught. It is in-born.
Just as no amount of studying the craft of writing and publishing will make you a good story teller, no amount of study in the use of graphic art tools (Illustrator, Photoshop, oil paints, watercolors, ...) will make you an artist.
If you have artistic ability then you can learn to create covers. If not, no amount of study will do more than give you the ability to create well crafted but ineffective and unattractive covers.