If someone has a problem they probably are seeking you out because you wrote the book, in which case you already have the job (unless HR discovers something bad) and the purpose of the interview is to convince you to take the job not the other way round! They already know you wrote the book and are hoping your pay isn't too much. This is the most valuable reason to write a book for most authors (there are other good reasons to write a book, but the rest typically don't produce $$$).
When the above doesn't apply you still need them to know you wrote the book because sometimes they will look up the book and a good book will overcome your bad interview.
The exception to the above is when you are bored of the topic and are interviewing for something completely different. In that case you hide it because they might assume you will leave for a better job in your field latter - after all you wrote the book in something else so you must be valuable.
When the above doesn't apply you still need them to know you wrote the book because sometimes they will look up the book and a good book will overcome your bad interview.
The exception to the above is when you are bored of the topic and are interviewing for something completely different. In that case you hide it because they might assume you will leave for a better job in your field latter - after all you wrote the book in something else so you must be valuable.