My personal point of view is as follows: No single set of rules, or even a universal definition, exists for what constitutes an "open book exam." It's possible that your instructor has determined that some resources are acceptable and others are not, depending on your situation. The rules established by your instructor for your specific course must be adhered to at all times. The term "cheating" refers to "any violation of the rules announced by your instructor."
Kylie
Student Ambassador, FAH
Coventry University