Obviously depends on what the book is, or what the app is. But a book is more impressive to me because if I took a random app on the App store, and a random book on Amazon, I would expect the book to represent more of a commitment on the author's part.
Why are you even asking this question? You come off as someone who is trying to "game" their image to appear more credible to others. If credibility is your sole aspiration, you'll probably never achieve it. Just make something, if it's good you'll gain credibility and if it's not then you'll just have to try again.
In case you're actually just curious, why not ask yourself? Do you respect authors of books you've read more than the authors of apps you use?
It was a 'shower thought'. I was reflecting on pretty much every podcast or written interview seems to start with the credibility of the guest. Author of a book seems to be the common theme. Creativity, philosophy, innovation that can be expressed in many ways - not only in writing a book. The purpose of my question was to survey what you all on HN thinks. I know my answer. What is yours?
It really depends on the quality of the book / app. Anyone can do either. But doing either with a high quality bar is something else again. I’d probably say it’s a tie.
If I’m hiring, though, a high quality app beats a book hands down. A book doesn’t tell me much about your ability to actually build production software.