What is your question?
There is no need for an app or program to read books posted online. Anything that lets you view such books is fine. But if you want to read a book that's posted online when you're not online, you have to either buy it or legally download it, if the author is giving it away. (Some do.) Then you can use whatever book reading program supports the version you downloaded, such as Kindle for PC or Kindle for Apple, both free.
I don't have a Kindle, but I buy and read books on Kindle using PC, laptop, iPad, and sometimes my phone.