The iTunes Store used to have a Power Search link in its page footers. You could choose to search specific types of content, and enter search terms in appropriate fields, such as Artist for music; Author for books; Actor for movies; etc. With iTunes 11, this link disappeared, but there’s still a way to get to it.
I had forgotten about this tip shared by Kirk McElhearn. If you want a faster way to access Power Search in iTunes using AppleScript, Doug Adams shows you how.