Look up the type of paper you are using in your book. Different papers have different thicknesses, and this will affect the PPI.
Determine the thickness of the paper you are using. This can be done by taking a caliper measurement of compressed paper or checking a standard paper thickness chart for the type of paper you are using.
Divide this number by two. You also can just enter it into a PPI calculator and get the results. The results from the calculator are the same as dividing the number in half by hand.
Use the resulting PPI in determining the width of your book. This is done using another calculation, where the number of pages in a book is divided by the PPI and a small margin for the cover thickness is added to the results.