For Apple devices, use the iBooks reader, which is free.
The ebook will open once you have finish downloading it. Click on the “Open in iBooks” button in the upper right corner. This will re-open the ebook in iBooks reader and a copy of the ebook will be stored in your iBook library.
Now you can access the ebook from iBooks, at anytime even when you are not online. This works on any iPad, iPhone, or iPod touch running the iBooks application.