"What is the meaning of "unleashed" on (these) book covers?" "What does "unleashed" mean on (these) book covers?" Well, your dictionary didn't exactly explain it properly. If something is on a leash (eg. a wild animal, or even more typically, a dog), it is constrained and controlled. If something is "taken off its leash", ie. unleashed, then it is released and freed.... but remember that it has a wild nature! The use of "unleashed" in your examples is for dramatic purposes.

It's just part of each book's title. It means nothing more than it's dictionary definition, although it is used metaphorically in those titles. You'd have to ask the authors why they gave the books those names, although I guess 'unleashed' implies that whatever the books describe is knowledge that was previously secret or unknown, or that the books contain so much information regarding their chosen subject that it's somehow dangerous to metaphorically 'remove the leash'. "Artistry Unleashed" could just have easily have been called "Unleashing Artistry", "Artistry Techniques Revealed" or something similar.