Answer: you can measure a book and estimate until 0.5 cm, so you can report 28.5 cm.
Step-by-step explanation:
You can measure, with a good level of precision, things that are bigger than several centimeters and smaller than the ruler.
If the object is too small, like the eraser of a pencil, the mark of 1 cm cannot tell the measure,
If the object is too big, like a a table, the ruler will not be a useful measurement tool because you will have to move the ruler and try to fix new marks on the table to keep the track.
You migh masure the lenght of your textbook which could be some 28 cm., taking into account that you will not be able to report measures like 28.1 cm or 28.9.
Neverthelles, you can estimate measures like 28.5 or 5.5 becasue the precision is half the distance between the marks.