The books should be available in your iTunes library on your computer as well. Assuming you have Sync Books selected under the Books tab for your iPad sync preferences with iTunes along with sync selected books, deselect the books you want removed from your iPad followed by a sync.
To delete a book direct from your iPad, select the Edit button at the top right ot the iBooks app. Select the book you want to delete followed by selecting the Delete button.
This is covered in the iPad user guide.