This is both a courtesy and a trap. It is a courtesy in that the examiner wants the candidate to work the examination just as he or she would approach a problem in professional practice. It is a trap in that the examinee may bring more reference material than they can practically use in the time allowed. This is a purposeful strategy on the part of those who prepared the examination. They know that in a time-constrained test, one can allocate only a limited amount of time for research.
Textbooks are very hard to use for research in a timelimited test.
They are usually narrowly focused, and tend to latch on to subtleties and nuances of a particular subject, spending pages and chapters on related material, making quick research very difficult. If you must bring textbooks, limit yourself to one or two and flag each with sticky tabs at those sections to which you want to refer. As a consequence, the proctor can not be counted on to provide answers to knotty technical issues. The proctor has been given instructions on how to administer the test and that is it. You are on your own once the clock starts.