iPads and iPhones don't easily receive downloads except from iTunes. Therefore, it is important that you make this order from a conventional computer (PC or Mac), not from your iPad or iPhone.
After clicking the "Buy Now" button, you will be taken to PayPal where you make the payment from a PayPal account or any major credit card.
Once payment has been made, you will be sent to a Download page.
Click on the Download button. The file, FORTY to LIFE for iBooks.epub, will download to your download folder. (You may wish to drag it onto your desktop if you don't know how to find that folder easily.)
Then send yourself an email with that file as an attachment.
Open that email in your iPad or iPhone and click on the attachment.
You will be asked if you wish to open it in iBooks. Click yes. (Once it opens in iBooks there is no need to save the email.)