Use Backspace key to delete to the left and Delete key to delete to the right. That is if you have a full keyboard layout.
On iBooks there isn't a delete key, keyboard is without two additional sections for navigation and for calculating. And it is annoying. I've spent a lot of elbow grease before I figured out that the trick is done with the 'fn' (funny) key - bottom row, last left key - to the left of Ctrl. Press and hold, hit Backspace and you delete to the right, same as with Delete key on full keyboaird layout.
Press and hold fn+Option, hit Backspace and you delete to the right word by word.
Hope this helps.
Sashura
Normandy