Tap Select at the top right (when in Library view). Tap the circle to the left of the book (list view) or the book cover (shelf view) to check it. Then tap Delete.
Note; if you are seeing a cloud symbol on or next to the book, the book is not actually on the iPad. It's a previously purchased book available for downloading. To hide these you can tap on Collections menu (Top Center) and turn on Hide iCloud Books at the bottom of the pop-down menu.
You can still see all purchased iBooks in the Purchased tab.