Q: How to download iBooks from iTunes Cloud

If using a computer's iTunes then what you can redownload will show in the Purchased section : either click on your name / icon towards the top right of iTunes and then click Purchased on the popup, or click on the Purchased link under Quick Links on the right-hand side of the iTunes store homepage. Click on the cloud icon on a book to redownload it.

If using a Mac with iBooks on it click on the Purchased link under Quick Links on the right-hand side of the iBooks Store homepage. Click on the cloud icon on a book to redownload it.

If using an iOS device then tap on the Purchased tab in the iBooks app and then tap on a book to redownload it.

If your books don't show (they won't show if the rights-holder has removed them from you country's store or if you've moved countries since buying them) then check to see if you've hidden them : Hide and unhide purchases in iTunes or iBooks on your Mac or PC - Apple Support