Thanks for the question. As noted [here](https://www.railstutorial.org/#print-edition), the print edition will be published by Addison-Wesley and is due out in Spring 2015. If you don't want to wait for that, I recommend the PDF on iPad, which is my preferred format for reading the book (even over print).
The MOBI version looks good on a Kindle, but I prefer the PDF on iPad because the PDF is print-quality. (This is due to limitations of the MOBI format and is not specific to the Ruby on Rails Tutorial.)