the metric system
You measure it.
Instruments such as calipers, rulers, and tape measures can be used to measure the width of a book. These instruments provide an accurate measurement of the book's width in either inches or centimeters.
With a ruler..... hahaha
Millimetres
Height by width and a ruler. :)
You would typically use inches to measure a paperback book. You can measure the length, width, and thickness of the book in inches to determine its size.
A centimetre should suffice.
A centimetre, although you could use a millimetre for greater accuracy.
Inches or centimetres are fine for a book's dimensions. Measure the width, height and depth to determine the volume. If you are talking about the length of the book's narrative, word count is probably the most accurate.
Base x Height x Width = Volume of any rectangular prism.
Most like a vernier caliper for accurate readings
Measure the length and the width. Or if you are feeling particularly energetic, then one of them and the diagonal.