In (mathematics) open-book exams I've taken, the books were essential. The highest scored solutions consisted of recognizing the problem, butchering equations for a few lines, and then pointing to the correct page in the correct book. The people who got highest grades were done in 15 minutes, though you were allowed to sit for at least 3 hours.

But, of course, to do that, you had to actually understand what it was all about, and you certainly couldn't learn that in 3 hours with all the books in the world.

That depends on where you live.