Back in the day, we just took these reference books at face value. Since they were, more or less, authorized, we trusted the authors without needing to see reprints or scans of memos and things. I still felt Asherman was prone to hyperbole ("we came close to lifting Gene Roddenberry upon our shoulders and carrying him out of the room"), but didn't have reason to doubt his version of the viewing of the first pilot.
One of the issues with those reference books is the authors aren't as thorough or meticulous researchers as @Maurice or @Harvey. Nor do a lot of them have journalistic skills. And often, hyperbole or, in the case of Mark Cushman aka Cash Markman, speculation gets passed off as fact.