The version in the iBooks store says it’s from the public domain text. This is likely from the Ambleside Online website so you might as well get the free ePub version from there and load it into iBooks.
Assuming you already have iBooks installed, just use Safari on your iOS device to go to the page with the ePUB files. Scroll down near the bottem and you should see the ePub links. Just touch the links and you should be prompted what you want to use to open it. Choose iBooks and it should do the rest.