Answer: 6 books
Work: Okay, The first thing I did was find out how much one book costs. To find that out, I divided $18/4, and the answer is $4.50 per book. Now, we add $4.50 until it's $27. I did that, and it was just 2 more books. (4.50 + 4.50 is $9, so add nine dollars to $18, and that would be $27. Then, divide 27/4.5, and you get 6, meaning the 6 books)
Sorry if this was confusing, but I hope I helped you. :)