The sentence does not use the whole expression, which is: To take a leaf out of someone's book.
I suppose the literal meaning of "leaf" here is "page".
The expression means to copy the good example set by someone else.
For example, imagine that you are spending far too much money on fuel for your car, while your friend X is using a good, cheap, bus service. If you complained to Y about the cost of using your car, Y might reply, Why don't you take a leaf out of X's book and use the bus?
Exactly, if you have authority to "take a leaf out of someone's book", you then "copy a good example set"; however, if you don't, I'm afraid you then "steal/crib/plagiarise someone else's idea/thought".
The expression is figurative. It looks to me like comsci and Alex_Murphy were showing you how the literal word "leaf", meaning a page or piece of paper, related to the eventual figurative expression "taking a leaf out of x's book".
Actually, you could use it literally, but I think most people would alter it slightly to make it clear that they did not intend the figurative meaning.
"I was short on paper for class today, so Alex let me borrow a leaf from his notebook" or "I hope you don't mind; I needed some paper so I took a couple of pages from your notebook."
This sounded funny to me, too, because the second half of the expression is missing. Whoever wrote this should have said something like "The ACFTU was taking a page out of the global anti-Wal-Mart movement's playbook, targeting the biggest and most high-profile company.
Note 1: As an American, I wouldn't say "leaf," although I would, of course, understand it if someone else used it.
Note 2: A playbook is a book in which a football coach (other sports, too?) describes all of the plays his team might run during a game. It is used to help new players learn what they'll need to know, as well as to remind current players of what they are supposed to know.
I understood that, JamesM, but I did not think it appropriate to introduce things like authority into this context, since I doubt the expression with 'leaf' would ever be used other than figuratively. As you suggested, in literal use one would say 'page'. Otherwise it could be misunderstood that 'leaf' is always synonymous with 'page', which it is not