First, I would be thankful that one of my executives was interested enough in a topic for which I had some responsibility that she/he would read a book about it. I know a lot of HR professionals that would like to have that problem. At a minimum, it creates an opportunity for you to build a personal & credible relationship with the executive (or not.) I would not interpret it as a lack of respect for my expertise.
What you should do if your executive gets a bad idea from reading a book depends on your relationship with him. (I say him because the problem you described is most often associated with male leaders.) If you have built a long & trusting personal relationship, I would say; “That is a dumb idea & here’s why.” That approach also assumes that in the past, you have reacted in a supporting manner to many of his ideas. If you have not established this kind of relationship, I might ask; “Are you interested in talking about how we might make this idea work at our company?” In the course of dialog about the practical aspects of executing the idea, the executive may come to realize it’s a bad idea. If you have presented all the objections and the executive still makes the decision to implement it, (assuming he has the authority to do that,) your job is to support it & do everything you can to make it work.
It is theoretically possible that an executive could get a good idea from reading a book. In this case, I would say; “Thank you” and then make it happen. It will certainly be easy to get her/his support for execution.
A quote from Van Doren hangs in my office and has been helpful to me. It says; “Bring ideas in and treat them royally for one of them may be the King.”