How to delete local books, in iBooks?

It's possible to add books you haven't purchased from iBooks store, or any text book or PDF document, it would not be a bad document manager which keeps them all in single convenient place, If only there was a way to categorize them. Trouble is if you ever wanted to delete a book, it's not easy at all. right clicking and choosing delete on these local books doesn't work. Unlike books downloaded from iBooks store; which when deleted show only a thumbnail with a little cloud icon over it, these pesky locals stick there rudely, and keep to occupy your HDD or SSD space. How to work around this bug?