I've taken a few electrical exams over the years of being an electrician. All of the exams are difficult and require the ability to index information while being timed. All electrical exams are open book.
The first exam I took was after 2 years into my apprenticeship. This test was for my Residential Wireman's License. It was a fairly easy test, but still 4 hours of daunting information that you have to try to find. I passed it on the first try with barely any knowledge of the NEC code book. I literally used the back index and searched for terms, then hunted through the book for the answers.
The next test I took was my Unrestricted Journeyman's License exam. This was a much harder test. I had a few more years of experience under my belt by this point, and had started using the code book more frequently. So I figured I'd just go into the testing center and take the exam. No problem right? Well...as it turns out the level of difficulty jumped up. I failed by ONE question! So I found a local Journeyman Electrician Test Prep course and paid a few hundred dollars to go through 3 days of intense codebook training. Boy did it work...the information I came across was stuff I'd never had known about had I not taken the class. They went deep into code knowledge, calculations, and electrical theory. Money well spent, as I passed the Journeyman Exam immediately after taking this course.
I took my Master Electrician Exam and passed my first go around, but I attribute that to studying for a solid year before taking it. I practiced with practice tests mostly. I also bought a bunch of Mike Holt's electrical study materials and almost every study guide and practice test/book I could find.