About the only thing we know for sure about this book, by using such a coarsely-divided ruler, is that it's between 2 and 3 cm high.
However, the trick to reading scales is to estimate one more digit than is shown on the ruler. This is where the uncertainty comes in -- our estimate isn't going to be exactly accurate.
We can see from the picture, though, that the book is somewhere around 2.5 cm high. We can improve on this by noticing that the book's height is definitely over the halfway mark between 2 and 3 cm... leaving only two possible choices, 2.6 cm or 2.7 cm. From here, it's your call -- how far over the halfway mark is it?