The answer is a significant one for the doctor. If he says no, then the attorney who is cross-examining him cannot ask him questions about statements in the textbook and whether he agrees or disagrees with the positions in the textbook.
If he agrees that the textbook is authoritative, that means that the trial attorney can now ask him questions about different statements in the book and whether he agrees or disagrees with them.
One of the more fascinating tactics to use at trial is bringing in an entire stack of seven or eight medical textbooks. Then, asking the doctor whether he recognizes each textbook as being an authority in its field.
After the first two or three textbooks that the doctor discards as not being authoritative, the jury quickly understands that no matter what textbook you pull out, the doctor is going to refuse to accept any textbook as authoritative.
Although the doctor will be prepared to give explanations about why these textbooks are not authoritative, the jury will begin to wonder what's going on.