Be careful with the units you use. The absolute uncertainty is given in millimeter, the height of the book in centimeter.
Before you even get out the calculator, though, try to estimate the answer roughly in your head, either mathematically or by visually picturing the size of 2 mm in comparison to 2.6 cm. A millimetre is a tenth of a centimetre, and the coefficients (the "plain numbers" in front of the units) are about the same. This suggests that the relative error should be somewhere around 10%.