Wicked is not good exam-time reading, as it's hard to put down, so you'll read it straight through and then move on to the next book (judging from my own experience).

Jonathon Strange and Mr Norrell is not good exam-time reading, as it's very very long and takes lots of time to read, but is good enough to make you want to spend your time on it rather than studying.

Your textbook is not good exam-time reading because you know you don't really want to read it even if it's interesting, because you're going to be assessed on it soon. And assessment can make even the most fascinating and fun material rather painful.