Yes, they get harder as you go through the book. You should expect, on the real thing, to get approximately half of the questions wrong overall (it may be 45%, it may be 60%, but closer to 50/50 than 90%+). The major determinant of score is the difficulty level of the questions.

So if you are getting about 60% of even the hardest ones right, that's good news - expect to see more of those than the easier ones on the test, b/c you will be scoring well and therefore will be given the harder questions selectively. DO NOT expect to get 95% of the questions right on the actual exam - that's NOT how it works.

As you get things right, the test will specifically try to give you things that you will then get wrong - as though it's searching for your limit - so you should expect to get a lot wrong on the test.