They are telling you that all of the answers on the test are in those 6 books.You should at least be familiar with them all.
I think the reason the do "open book" is because these things change so often that you need to be able to find the latest rules in the book.
I would tend to agree with Reno though. If you really know your stuff, you should be able to get a passing grade with a book or two that have the tables that you may not need to memorize 100%.
We use 310.16 all the time but I know you might get me with some value I don't use that often and that is the kind of thing test writers like to throw at you.
They want you to be able to look things up and interpret all of the factors you need to include.
An example is the 1hp 120v motor. What is the minimum size copper wire and the max breaker? You need at least 3 tables to get that answer.
