Q: How can you delete a book  from iBook ?

nope.  not for me.

you cant delete from ibook but you can hide it

http://support.apple.com/kb/ht4919

Yes, you can delete ebooks, from iBooks. Please stop saying it isn't possible.

I would love it if someone could clease explain how I can delete unwanted books from iBooks. I am using Yosemite on a Mac Air.

The problem is that I have a large collection of books, many of which I've read and now just want to dump. I tried using "Control-Delete" in iBooks, but the option to "Delete" doesn't appear.

PS I don't know if it makes a difference, but many of these books are not purchased from Apple, just downloads from Project Guthenberg.

The shortcut is not Control Delete, it's Control click on the book you want to delete, and a dialogue box should appear to delete or, you can click on the book you want to delete, and using the Menu Bar > Edit > Delete.

Thanks Picas - I didn't explain well. I did actually "Control-click" then look for "Delete" in the Dialogue box. The reason it wasn't there is that apparently I am not allowed to delete a book that is on iCloud as well as a purchased book - I can only hide it. I hope this also isn't suffering from the same lack of clarity as my earlier description.....

Just to clarify, you can delete a purchased Apple book from iBooks, but a copy will remain in iCloud. If you do not wish to see those books select View >  Hide iCloud books.

But wait, there's more. Another way to hide books is to select Purchased in Quick Links, select a book and a little x appears top left of book. Select the x to hide the book. This book will not show even if you have View > Hide iCloud books deselected. You can unhide these books by selecting Account in Quick Links > iTunes in the Cloud > Hidden Purchases.