What’s he going to do? Read his book during interviews or debates? He needs to be able to clearly articulate his positions on any occasion with a depth of knowledge and authority, not just refer to books.
It’s not about referring to books.. you have to realize that he is a political outsider so he is relying on much of the same resources that we do to form opinions... writing the book means he has taken time to meet with the people that know and he has formed an opinion based on that... if the book is more rhetoric then he has a problem, but if he details specific plans that will form the basis of his campaign moving forward I don’t see a problem.