Summarize:

You can select between "All" or "Not on This iDevice". This looks like a little cloud, and is to the right of the book's title. After downloading, your book will be selectable from the iBooks main menu.
Ensure you are logged into your device with your Apple ID. Open the iBooks app. Tap the "Purchased" tab. Find the previous purchase. Tap the "Download from iCloud" button. Open your book.