Generally I would recommend ebooks. Most people rarely have the time to shop for hard books nowadays. I have been busy lately, but I was able to successfully Publish an ebook I've been working on for the past three months with the help of a ghostwriter. This single book has fetched me over $10,000 on Amazon since it was published. Ebooks are great especially when you write them personally, or with the help of professional freelancers. Moreover, publishing your ebook online with a well optimized listing is a continuous source of passive income.
I was just curious about the book.