Whether someone needs a PhD to write a textbook is common knowledge and a matter of publishing standards rather than statutory law, requiring no formal citation.
The claim addresses whether holding a PhD is a legal prerequisite for authoring a textbook. This is a matter of common knowledge regarding publishing practices and the lack of government licensing requirements for writing books. None of the provided retrieved papers deal with legal qualifications for textbook authors, making them entirely tangential. The statement is trivially true (there are no laws requiring doctoral degrees to write books), so it falls under COMMON_KNOWLEDGE without requiring citations.