Compare following expressions in English used for the same purpose of asking someone sitting next to you to pass a book:
‘Pass the book.’
‘Can you pass the book?’
‘Could you pass the book?’
‘Could you pass the book, please?’
If you use the first expression, you’ll come across as rude. The person may still pass the book, but with a frown on your temerity to ‘order’ him.
The second is OK.