A centimetre should suffice.
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With a ruler..... hahaha
Measure the length and the width. Or if you are feeling particularly energetic, then one of them and the diagonal.
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.
I will measure a book with grams
A centimetre, although you could use a millimetre for greater accuracy.