Two ways, until somebody smarter comes along with a better way.
1. If your computer is a Mac. Then search txt to iBook, scroll down the results page to the article ' pages to ePub.' Using this approach, you open the txt file in pages, then export it in ePub format, then sync to your iPad.
2. If you are not a Mac guy ( I am not either), you can e mail the book to yourself and open it in pages, docs2go, good reader, or other available readers.
I'm sure there is a way to get it into ePub format easily, but I have not found it yet.