This one is a judgement call. The height of the book is definitely more than half the way to the 3 cm mark, so 2.4 cm and 2.5 cm are out. However, the next digit might be a 6 or a 7, depending on how you read the ruler.
Generally speaking, the last digit quoted in any measurement is the one that's somewhat uncertain -- it might be off by ±1, ±2, .... . In our example, the last digit is uncertain by ±1.