Remember when you were a kid and you were told that when you read a word you didn't know you should look it up in the dictionary? Yeah, I never followed that advice either But I'm using it like crazy in iBooks. It's so easy, you check on words you know but want to see why the author is using it that way. And I find myself wishing Apple would build this into the OS on a system-wide basis because I keep trying to look up words in Safari, etc. using the same method.