Again, this is a judgement call. The ruler is only marked to the nearest centimetre, which means we can estimate a value for tenths of a centimetre. However, there will be some uncertainty in this -- after all, it's just a guess (but an educated one at that).
If the conditions are ideal -- the markings on the ruler are clearly visible, there's lots of light in the room, the ruler is held perfectly still, you're looking straight-on at the book and ruler -- the uncertainty will probably be 0.1 cm. Conditions rarely are ideal, though ... so a more realistic estimate might be 0.2 cm for the uncertainty.
To summarize, the amount of uncertainty in a measurement isn't set in stone. It depends on the situation and the conditions at the time. It's your decision, but be reasonable.