If you don't buy the author's book, you apply for a visa, and the staff doesn't stop you, that's fine. The author is not a fool, even if he is a little embarrassed, he will not refuse the visa, otherwise both sides will feel bad. Explain to the author and then express that you like his work very much. I hope the author can understand, which is of course no problem. It is also suggested that you bring a pen and paper when you visit the exhibition, or you can use it in this case!